Add support for Core Data model files

[brentleyjones]

Part of #208.

[brentleyjones]

This currently has an issue related to codegen. For an executable bundle (e.g. app, dynamic framework, extension, etc.), the data model should be in the Compile Sources phase. This will create any necessary codegen and still bundle the resources. For other targets (e.g. resource bundle) it should be in the Bundle Resources phase (as it's invalid for iOS resource bundles to have an executable) and also the Compile Sources phase of the associated static library/framework.

This results in weird behavior though, because the Bundle Resource phase will complain about generated code. Disabling codegen gets rid of all of this, but it needs to be enabled on the entities for apple_core_data_model to work. Ohh, and apple_core_data_model generates these sources as well, so we have to deal with the duplication somehow.

So this is what I'm thinking: we post-process the .xcdatamodel/contents files, removing the codeGenerationType key. We keep the xcdatamodeld in Bundle Resources phase for all targets (versus Compile Sources). We allow the sources from apple_core_data_model to land where they land. This should mimic how Bazel behaves.

[brentleyjones]

Rebased. I'll address the issue above at a later time. I'll open an issue for it.

[erikkerber]

This combined with the recent signing fix has us building and running on the simulator 🎉