expose protoc-gen-grpc-swift as a gRPC generator

[crockeo]

Up until now rules_swift only exposed protoc-gen-swiftgrpc, the 0.x gRPC generated for Swift. This PR adds support for the 1.x gRPC generator, protoc-gen-grpc-swift. protoc-gen-grpc-swift generates code that depends on GPRC and NIO, so most of the code in this PR is setting up builds for GRPC and NIO. Not sure if this needs to live in rules_swift, but it's good enough for now 🙂

Some remaining todos:

• [ ] Audit all of the additional builds to ensure they have their required dependencies (cross-reference w/ upstream builds)
• [ ] Create builds for CNIOWindows and CNIOLinux
• [x] Fix issue where generated .grpc.swift files can't compile because they don't import their accompanying .pb.swift
• [x] Only compile the relevant set of support libraries (SwiftGRPC for swiftgrpc, GRPC/NIO for grpc-swift)

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[crockeo]

@googlebot I signed it!