Add scheme build architecture overrides

[arhxam]

Summary

• expose Xcode's scheme-level architecture override through build.buildArchitectures
• support matchRunDestination, universal, and useTargetSettings in parsing, encoding, generation, and documentation
• preserve XcodeGen's existing useTargetSettings behavior when the option is omitted

Resolves #1524.

Test plan

• swift test (76 tests passed)
• swift test --filter SchemeGeneratorTests/testSchemes
• swift test --filter SpecLoadingTests
• generated a project with all three modes and verified the resulting scheme XML uses Automatic, All, and no override attribute, respectively

This pull request was prepared with assistance from OpenAI Codex. I reviewed and tested the changes locally.