Implement Query (part 1) and add improved support for Futures

[darinf]

Adds support for Query.getDocuments. A follow-up PR will add support for the various other Query methods.

Most of this PR is about introducing new infrastructure for working with Firebase C++ Future instances:

• Avoids needing to call Wait on the Future to block a thread for completion.
• Just rely on the underlying OnCompletion machinery of Firebase that already does the right thing.
• Add FutureProtocol and conformance through a new C++ subclass of Future<R> in the swift_cxx_shims namespace.
• Enable sharing of code for Future handling.
• Update DocumentReference.getDocument to use new Future handling.

A follow-up PR will revise setData to use new Future handling as well.

The async method variants are built on top of closure based APIs to better match the expectations of callers that are coded against the Obj C API. Arc needs these completion-callback variants, and we can just implement the async versions as a trivial layer around those.

Switches from @_spi(Error) to @_spi(FirebaseInternal) since we are exporting now more than just FirebaseError as SPI from FirebaseCore.

Also updates the DocumentReference.get to .getDocument to match Firebase Swift API documentation and expectations of callers that are coded against the Obj C API.

This approach makes the OnCompletion_SwiftWorkaround patch unnecessary. We can eliminate that once setData is converted over to this new mechanism.

[brianmichel]

LGTM just had a few small questions

[compnerd]

Would be nice to split this up into two separate PRs - one for the future changes and one for the FireStore Query additions.

[darinf]

Would be nice to split this up into two separate PRs - one for the future changes and one for the FireStore Query additions.

Yeah, I don't think that is worth it. The point was to demonstrate the use of the new Future setup. I will have a follow-up that adds a lot more meat to the Query implementation.

[darinf]

I did some more testing and found a substantial issue with the callbacks. Unfortunately the Firebase C++ SDK is not running callbacks on the main thread, which is in contrast to the Obj C API. I'll need to fix that. As a result I may invert the setup so that the completion callback API is actually built on top of the async API. That way the async API does not need to pay the cost of hoping thru the main thread.

[compnerd]

Thanks for the clean ups with the protocol, it definitely feels much better.