Add Bigtable Client Conformance Tests

[meagar]

The Bigtable team maintains a suite of language-independent "conformance tests", which verify some common edge-cases behave correctly across all of the Bigtable clients.

We would like to add conformance tests for the Ruby client. This is accomplished by authoring a thin proxy, which implements the CloudBigtableV2TestProxy service. The goal of the proxy is to idiomatically use the client library to handle the requests it receives, not to jump through hoops to make the tests pass; in this way, we can verify the idiomatic library use produces correct results.

The tests themselves are implemented in Go in https://github.com/googleapis/cloud-bigtable-clients-test. These tests spawn a mock Bigtable instance, so that the test runner can verify correctness both in the proxy's responses, but also in the client's outbound calls to Bigtable itself.

[meagar]

Apologies, I'm backfilling this issue, the work has already been done in #25680 before I realized all PRs should have an issue assigned.