Add StoreKit Configuration support to Xcode Schemes.

[yanks]

This change satisfies requirements of #3130, allowing the user to supply a relative path to a storekit configuration file based on the workspace root:

xcodeproj(
<snip>
    xcschemes = [
        xcschemes.scheme(
            name = "MyApp",
            run = xcschemes.run(
                launch_target = ":MyApp",
                storekit_configuration = "path/to/StoreKitConfig.storekit",
            ),
        ),
</snip>
)

The result is this written to the relevant xcscheme:

      <StoreKitConfigurationFileReference
         identifier = "../../path/to/StoreKitConfig.storekit">
      </StoreKitConfigurationFileReference>

A few notes:

• I'm pushing this out a bit early, because I couldn't figure out a great way to resolve a label to a given filegroup to a reasonable path relative to the project root. xcschemes_json encodes as a string into the rule, and it seems a bit unwieldy to have to resolve this issue for something that will never be modeled within bazel anyway (as this is an Xcode-only feature). I would love input on this! I think the tradeoff of just requiring the file to be relative to the workspace root to be sufficient, but happy to discuss.
• There are a few more tests I'd like to write, and some guardrails on the file itself (project generation should fail when it can't resolve the path to storekit file)

Here's a screenshot of this working within our project:

[brentleyjones]

Approve only to get CI to run.

[brentleyjones]

Please make this alphabetical.

[brentleyjones]

Same, and everywhere.

[brentleyjones]

Suggested change

	}
	}

[brentleyjones]

We haven’t needed this until now. I question if we still need it. Is there a way to make sure it comes in correctly from Starlark instead?

[yanks]

I struggled with this a lot. I suppose I could relativize this within starlark and provide a relative path there, but there is no great way to get a true path to the file via the rule itself. The schemes are encoded via json and supplied here: https://github.com/MobileNativeFoundation/rules_xcodeproj/blob/main/xcodeproj/internal/xcodeproj_incremental_rule.bzl#L835

And other things within the scheme match up based on identifier encodings, but this requires a path that is relative to the resultant scheme directory itself in the generated xcodeproj from what I can tell. I am happy to do it differently, but I think the outcome ends up being something considerably more complex. I'm not particularly experienced in working with rules_xcodeproj, so after banging my head on this for a little bit I threw up my hands and went with this. Open to suggestions!

[brentleyjones]

Mainly we want to be able to "cache" things in Bazel. Doing this work in the generator means it's re-done every time the generator runs. I'll leave it up to the other @MobileNativeFoundation/rules_xcodeproj-maintainers on what they think about this.

Only approved to run CI.

[chrisballinger]

Oh perfect, thank you for diving into this!