[file_selector] Migrate FileSelectorApi to Dart

[adpinola]

This PR includes the changes in #6518 and in #6539.
This exposes a new FileSelectorApi that will replace the one generated by pigeon.
This will be integrated with the next PR.

Things we have done:

• Add FileSelectorApi
• Add SelectionOptions class
• Add DialogWrapperFactory for testability

Issue: [file_selector_windows] Migrate CPP implementation to Dart. #112212

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[adpinola]

Closing this because it is opened in the main repository
[file_selector] Migrate FileSelectorApi to Dart #6546