Emit warning if executable in resource manifest can't be found

[SteveL-MSFT]

PR Summary

• Update build script to not copy WindowsPowerShell resource manifest on non-Windows, creates noise for testing
• During resource discovery when loading the manifest, validate that the executable specified for the operation can be found otherwise emit a warning message
• Remove outdated comment that get is always supported

[Copilot]

Pull Request Overview

This pull request enhances resource discovery by validating that executables specified in the resource manifest are available, emitting a warning if they're not found.

• Added a new function (verify_executable) to check executable presence for various operations.
• Updated Cargo.toml to include the which dependency.
• Updated locale files with a new warning message for missing executables.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

File	Description
dsc_lib/src/discovery/command_discovery.rs	Added executable validation via verify_executable for each resource operation and integrated SchemaKind handling
dsc_lib/Cargo.toml	Added the which dependency to support executable lookup
dsc_lib/locales/en-us.toml	Added a new locale message 'executableNotFound' for warning messages

Comments suppressed due to low confidence (1)

dsc_lib/src/discovery/command_discovery.rs:592

• Consider adding unit tests for 'verify_executable' to ensure that the warning is correctly emitted when an executable is not found.

fn verify_executable(resource: &str, operation: &str, executable: &str) {