Add a build manifest build step

[alfonso-noriega]

WHY are these changes introduced?

To support the generation of build manifests for UI extensions, which will be used to map extension points to their compiled assets.

WHAT is this pull request doing?

Adds a new build_manifest step type to the build system that can:

1. Generate a single manifest JSON file with asset mappings
2. Iterate over extension points to produce per-target manifests
3. Support various filepath resolution strategies:
   • Literal strings
   • Values from TOML configuration
   • Composed paths with prefix/filename components

The implementation includes:

• Full test coverage for all resolution scenarios
• Support for optional assets that are silently skipped when not available
• Handling of nested arrays in configuration

How to test your changes?

1. Create an extension with multiple extension points
2. Add a build_manifest step to its build config
3. Build the extension and verify the manifest is generated correctly
4. Check that the manifest contains the expected asset mappings

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[alfonso-noriega]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add a build manifest build step #6869 👈 (View in Graphite)
• Replace hosted_app_home build mode with copy_files mode #6868
• Refactor extension build process to use modular build steps #6867
• Abstract build steps to externalize the build configuration #6866
• Asset pipeline flow for hosted static app #6806 : 1 other dependent PR (#6842 )
• main

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