Live tests: automate test runs against sandbox accounts.

[clnoll]

Now that we've added connections to Cloud for our sandbox accounts, we can move forward with automating live tests.

Implementation

• Add a list of connection IDs against which to test to each connector's metadata.yaml. These sandboxes should come from the integration-test-sandboxes workspace here. We can match up the source:connections using airbyte-api and automatically populate the metadata.yaml for the connector.
• Update the GHA workflow for regression tests to add a pull request option. When regression tests are run in this context, they'll pass in the connection ID from a sandbox account.

Acceptance criteria

• Live tests run automatically on PRs, against sandbox accounts only.
• The outcome of the test run is visible in a status check.