feat: Add Kafka Connect Cloud Bigtable sink connector

[prawilny]

This PR adds Kafka Connect sink connector.

The code is to land in a different repository, but the repository hasn't been created yet, so we bring the code for early review here.
The fact that it's targetting another repo is the reason of the following:

• modification of Github Actions CI so that it executes the sink's tests instead of adding it as a new maven submodule and plugging it into existing CI
• the code is not a submodule in the root pom.xml nor does it use any information from outside of the directory
• maven plugins' config is duplicated
• there is another copy of the license in the directory

Things yet to be done (in future PRs):

• Logical types support
• More comprehensive integration tests
  • they might include more detailed compatibility checks against the Confluent sink
• Use of kokoro in CI

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/workflows/ci.yaml