Upgrade grpc-swift to 1.21.1

[kev-the-dev]

Upgrade grpc-swift to 1.21.1, along with some of its transitive dependencies. This new version contains bug fixes and and access to new APIs, such as the GRPCChannelPoolError struct being publicly exported. The versions of these transitive dependencies were pulled directly from grpc-swift's package.swift.

This also required some changes to the grpc-swift build overlay, since grpc-swift split up into some sub-libraries instead of one monolith library. This was done iteratively, adding in these new libraries until the grpc-swift build passed.

Testing

In a project, can build swift packages which use grpc and make use of new features in this version of grpc-swift.

[google-cla[bot]]

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[kev-the-dev]

Standby, working on figuring out the appropriate way to submit the CLA

[kev-the-dev]

CLA complete and passed

[mattrobmattrob]

Hey, @kev-the-dev. Can you provide more detail on the PR description about why you're doing this update? And can you fix the failing CI jobs?