Enable support for AEGs

[brentleyjones]

(cherry picked from commit 8a4310e)

[brentleyjones]

We will need to apply bazelbuild/rules_apple@3da7b31 in rules_apple before landing this. as well as some other toolchains/AEG stuff in general.

[brentleyjones]

Current failures are unrelated to PR, but I don't feel safe merging this while they are there.

[brentleyjones]

Also looks like we need to set toolchains for the proto stuff. Upstream defines swift_proto_toolchain, and I think we have to do something similar. cc: @AttilaTheFun.

[AttilaTheFun]

@brentleyjones I'm not familiar with Automatic Extension Groups but you're welcome to modify the swift_proto_library rule to support them.

However, longer term I was thinking it would be more maintainable to refactor them into a macro with a source-generating rule which passes the generated sources into a regular swift_library.

This is the approach I took at Snap for our new Swift dependency injection system and it has been working well for us so far.

There are a couple of tradeoffs to this approach, though, and it would be a source-breaking change so we would have to retain the old behavior with some kind of legacy_mode flag.

[brentleyjones]

Ignore the AEGs portion, I was more calling out the toolchain-ification of the proto rules in upstream and how that probably applies to us as well (and in general using toolchains is required for AEGs to work).