encoding: Add a test-only function for temporarily registering compressors (#8587)

encoding: Add a test-only function for temporarily registering compressors (#8587)

Fixes: https://github.com/grpc/grpc-go/issues/7960
This PR adds a function that allows tests to register a compressor with
arbitrary names and un-register them at the end of the test. This
prevents the compressor names from showing up in the encoding header in
subsequent tests. Previously, tests were using the name of the existing
compressor "gzip" and re-registering the original compressor to
workaround this problem.

RELEASE NOTES: N/A