feat(tests): [Bigtable] add conformance tests and proxy

[bshaffer]

b/372508972

• Exposes classes for https://github.com/googleapis/cndb-client-testing-protos/blob/main/google/bigtable/testproxy/test_proxy.proto in a proxy
• Uses roadrunner.dev to implement a gRPC server in PHP
• Tests are passing, but with the caveat that a list of known_failures have been added - they are detailed below.

Fixed Tests

These are tests which have been fixed by small changes to the library

• TestMutateRows_NoRetry_NonTransientErrors - Fixed by adding "index" to$rowMutationsFailedResponse. See this line. This should not be a breaking change, but it is a change, so it should be confirmed.
• *_Generic_CloseClient - these are fixed by ensuring the appProfileId is propagated correctly to the requests. This functionality worked in V1 but was broken in the V2 release, and is now fixed (see this line for an example)

Failing Tests

• TestReadRows_NoRetry_OutOfOrderError and TestReadRows_NoRetry_OutOfOrderError_Reverse - no error is thrown (an error should be thrown which contains "increasing" and "decreasing" in the error message, respectively). This seems to be an issue which requires fixing.

• TestReadRows_Retry_PausedScan, TestReadRows_Retry_StreamReset, and TestReadRows_Retry_LastScannedRow - I believe what's happening here is there is an error because the retry request doesn't contain "RowRanges" as expected. This may also be an issue which requires fixing.

• TestReadRows_Generic_DeadlineExceeded and TestReadRow_Generic_DeadlineExceeded - failing because the deadline exceeded error is being suppressed by the retries. This seems to be WAI.

• TestMutateRows_Generic_DeadlineExceeded - Failing because the Deadline Exceeded exception is suppressed and added to the BigtableDataOperationException. This seems to be WAI.

[mutianf]

LGTM but can we remove a few tests from known failures? Thank you!

[mutianf]

Can we remove TestReadRows_NoRetry_OutOfOrderError, TestReadRows_Retry_StreamReset, TestReadRows_Retry_PausedScan and TestReadRows_Retry_LastScannedRow from this list? So we don't forget about them :) We don't have to mark conformance test passing as a required step for presubmit for now.