Allow running integration tests on an external PR (#404)

Add an input parameter to the integration tests workflow that allows you to specify a PR number (and optionally a commit hash within that PR) to run tests on.

This lets admins manually trigger integration tests on an external PR. To test an external PR, use workflow_dispatch from the Actions tab to trigger the integration tests workflow, and then specify the PR number and hash in the input parameters. (You can use this to test any PR as an alternative to labeling.)

Author: jonsimantov