XCFramework Support via CocoaPods

**Describe the bug**

When importing gRPC via CocoaPods, it pulls in `abseil`, which is only available as a source pod. This means when we import gRPC into our app, we have to rebuild `abseil` every time we do a clean build. This adds a significant amount of time to our build times.

Apple has a native binary format called an XCFramework that would allow for `abseil` to be distributed as a binary pod, and CocoaPods plugins like [cocoapods-pack](https://github.com/square/cocoapods-pack) exist to produce XCFrameworks from podspecs. If `abseil` was distributed as an XCFramework, we would be able to import it as a binary pod, which would speed up the builds for our app. This would be a boon for every iOS app that imports `abseil`.

**What build system are you using?**

CocoaPods and `xcodebuild`

**Additional context**

I have a sister issue open for `gRPC` at https://github.com/grpc/grpc/issues/28749