Preserve nested target attribute dictionaries

[sergeyospanov]

Summary

• preserve nested target attributes such as SystemCapabilities as native PBX dictionaries instead of quoted Swift dictionary descriptions
• add a generator-level regression test for the HealthKit capability shape from the issue

Testing

• swift test (77 tests, 0 failures)
• swift test --filter PBXProjGeneratorTests/testNestedTargetAttributesAreGeneratedAsDictionaries
• generated the issue's YAML shape with the CLI and verified that project.pbxproj contains a native SystemCapabilities dictionary

Resolves #1637

[sergeyospanov]

After opening this PR, I found #1639, which addresses the same nested-dictionary conversion in ProjectAttribute.init(any:). Sorry I missed the overlap.

This PR adds focused SystemCapabilities/HealthKit regression coverage in PBXProjGeneratorTests, while #1639 covers the conversion through ProjectGeneratorTests.

@yonaskolb, which direction would you prefer? I'm happy to close this PR, or to contribute any useful test coverage to #1639 if its author and the maintainers want it. I won't make further changes here until you advise.