Add a Github workflow action to run Bigtable conformance tests

[meagar]

I've authored Bigtable Client Conformance Tests (#25680) a for the google-cloud-bigtable client, and we would like to run these tests for pull requests that modify files under that directory.

There is prior art for the Java bigtable client, the Node client and many others, but these repos are smaller and scoped to only the Bigtable client library, while the google-cloud-ruby repo contains many client libraries for many products.

I'd like to understand how I should add a Github workflow for only the google-cloud-bigtable folder, possibly this is as simple as making a new google-cloud-bigtable-conformance.yaml file in the .github/workflows folder, and using something like this suggestion from Stack Overflow to restrict the workflow to run on changes to the bigtable folder?

on:
  push:
    paths:
      - folder1/**

[dazuma]

You can model after the existing workflow https://github.com/googleapis/google-cloud-ruby/blob/main/.github/workflows/storage-retry-conformance-test-against-emulator.yaml for now. (In the fullness of time, we'll probably want to shard the bigtable handwritten client, along with your conformance test and any other similar bigtable-specific infrastructure, info a separate repository.)