Enable embedding install location options in apphost (#104749)

- Add placeholder value in `apphost` for .NET install search options. Format:
  - `<search_location_flags> /0 <app_relative_dotnet_path>`
- Make `apphost` conditionally look at app-local, app-relative, environment variables, and global locations based on configured behaviour
  - Default (placeholder not changed) is to look at app-local, environment variables, and global locations.
- Update error and tracing messages to include information about any configured search options
- Allow specifying search options in `HostWriter.CreateAppHost`
- Add unit/integration tests

Part of https://github.com/dotnet/designs/blob/main/proposed/apphost-embed-install-location.md.
There still needs to be a corresponding change on the SDK side to allow configuration via `AppHostDotNetSearch` and `AppHostRelativeDotNet` properties.

docs/design/features/host-error-codes.md

@@ -31,7 +31,7 @@ Note that the exit code returned by running an application via `dotnet.exe` or `
 | `LibHostInvalidArgs`                  | `0x80008092` | `-2147450734`          | `146`         | Arguments to `hostpolicy` are invalid. This is used in three unrelated places in the `hostpolicy`, but in all cases it means the component calling `hostpolicy` did something wrong: <ul><li> Command line arguments for the app - the failure would typically mean that wrong argument was passed or such. For example if the application main assembly is not specified on the command line. On its own this should not happen as `hostfxr` should have parsed and validated all command line arguments. </li><li> `hostpolicy` context's `get_delegate` - if the requested delegate enum value is not recognized. Again this would mean `hostfxr` passed the wrong value. </li><li> `corehost_resolve_component_dependencies` - if something went wrong initializing `hostpolicy` internal structures. Would happen for example when the `component_main_assembly_path` argument is wrong. </li></ul> |
 | `InvalidConfigFile`                   | `0x80008093` | `-2147450733`          | `147`         | The `.runtimeconfig.json` file is invalid. The reasons for this failure can be among these: <ul><li> Failure to read from the file </li><li> Invalid JSON </li><li> Invalid value for a property (for example number for property which requires a string) </li><li> Missing required property </li><li> Other inconsistencies (for example `rollForward` and `applyPatches` are not allowed to be specified in the same config file) </li><li> Any of the above failures reading the `.runtimecofig.dev.json` file </li><li> Self-contained `.runtimeconfig.json` used in `hostfxr_initialize_for_runtime_config`. Note that missing `.runtimconfig.json` is not an error (means self-contained app). This error code is also used when there is a problem reading the CLSID map file in `comhost`. </li></ul> |
 | `AppArgNotRunnable`                   | `0x80008094` | `-2147450732`          | `148`         | Used internally when the command line for `dotnet.exe` doesn't contain path to the application to run. In such case the command line is considered to be a CLI/SDK command. This error code should never be returned to external caller. |
-| `AppHostExeNotBoundFailure`           | `0x80008095` | `-2147450731`          | `149`         | `apphost` failed to determine which application to run. This can mean: <ul><li> The `apphost` binary has not been imprinted with the path to the app to run (so freshly built `apphost.exe` from the branch will fail to run like this) </li><li> The `apphost` is a bundle (single-file exe) and it failed to extract correctly. </li></ul> |
+| `AppHostExeNotBoundFailure`           | `0x80008095` | `-2147450731`          | `149`         | `apphost` failed to determine which application to run. This can mean: <ul><li> The `apphost` binary has not been imprinted with the path to the app to run (so freshly built `apphost.exe` from the branch will fail to run like this) </li><li> The `apphost` is a bundle (single-file exe) and it failed to extract correctly. </li><li>The `apphost` binary has been imprinted with invalid .NET search options</li></ul> |
 | `FrameworkMissingFailure`             | `0x80008096` | `-2147450730`          | `150`         | It was not possible to find a compatible framework version. This originates in `hostfxr` (`resolve_framework_reference`) and means that the app specified a reference to a framework in its `.runtimeconfig.json` which could not be resolved. The failure to resolve can mean that no such framework is available on the disk, or that the available frameworks don't match the minimum version specified or that the roll forward options specified excluded all available frameworks. Typically this would be used if a 3.0 app is trying to run on a machine which has no 3.0 installed. It would also be used for example if a 32bit 3.0 app is running on a machine which has 3.0 installed but only for 64bit. |
 | `HostApiFailed`                       | `0x80008097` | `-2147450729`          | `151`         | Returned by `hostfxr_get_native_search_directories` if the `hostpolicy` could not calculate the `NATIVE_DLL_SEARCH_DIRECTORIES`. |
 | `HostApiBufferTooSmall`               | `0x80008098` | `-2147450728`          | `152`         | Returned when the buffer specified to an API is not big enough to fit the requested value. Can be returned from: <ul><li> `hostfxr_get_runtime_properties` </li><li> `hostfxr_get_native_search_directories` </li><li> `get_hostfxr_path` </li></ul> |