Support `objc_library`

[brentleyjones]

• [x] https://github.com/buildbuddy-io/rules_xcodeproj/issues/68
• [x] https://github.com/buildbuddy-io/rules_xcodeproj/pull/147
• [x] Support alwayslink: #608
• [x] Support pch: https://github.com/buildbuddy-io/rules_xcodeproj/pull/280
• [ ] Support runtime_deps
• [x] Support sdk_dylibs: #372
• [x] Support sdk_frameworks: https://github.com/buildbuddy-io/rules_xcodeproj/pull/278
• [ ] Support sdk_includes
• [x] Support weak_sdk_frameworks: https://github.com/buildbuddy-io/rules_xcodeproj/pull/278

[maxwellE]

@brentleyjones I think some of this issues' sub tasks are now completed

[brentleyjones]

I checked off the ones I'm sure of. We need examples of the remaining ones.

[brentleyjones]

While we don't have an example for runtime_deps, unless a bug is reported, I'm claiming this is done.