Improve the xDS bootstrap package

[easwars]

We currently have a bunch of xDS bootstrap packages:

• xds/bootstrap
  • This currently only contains code dealing with channel credentials functionality in the bootstrap file.
    • This should be moved to xds/bootstrap/credentials.
• internal/xds/bootstrap
  • This currently has the bootstrap parsing code used by production code. The parsing functionality is also used minimally by test code.
  • All types defined here like Config, ServerConfig, Authority, ChannelCreds etc should support the following functionality:
    • A MarshalJSON method to output marshaled JSON bytes
    • A UnmarshalJSON method to read JSON and update the internals of the type
    • A Equal method to avoid implementing custom equality functions in tests
    • A String method to output human readable contents for debugging
  • This should be moved to xds/bootstrap
• internal/testutils/xds/bootstrap
  • This has bootstrap parsing functionality used by test code
  • This should be removed and we should get the test code to be functionality from xds/bootstrap

[easwars]

Based on offline discussions, we think it makes sense to keep the bootstrap functionality in internal/xds/bootstrap since it is being used from outside of the xDS tree. Also, putting it in xds/bootstrap will make the API public which is not desirable.

[easwars]

A bunch of cleanup happened in https://github.com/grpc/grpc-go/pull/7299.