Add Equatable and Hashable property retention

[horia-openai]

Summary

• Add retain_equatable_properties and retain_hashable_properties configuration options and CLI flags.
• Retain stored properties for value types that conform to Equatable or Hashable, including inherited protocol conformances.
• Add fixtures and retention tests covering both options.

Motivation

Swift can synthesize Equatable and Hashable conformances from stored properties without producing explicit references in index data. That can make properties appear unused even though changing or removing them changes synthesized equality or hashing behavior.

This keeps the behavior opt-in, similar to the existing retain_codable_properties setting, so projects can choose the broader retention behavior when synthesized protocol requirements are part of their API surface.

Fixes #869.
Fixes #1028.
Related to #721.

Testing

• swift test --filter "RetentionTest/testRetains"
• swift test
• swiftformat --quiet --strict .
• swiftlint lint --quiet --strict
• bazelisk run //bazel/dev:buildifier.check
• git diff --check

[horia-openai]

@ileitch This is ready for review, please take a look.