FR(OwlBot): Add a label to automatically update .Owlbot.lock.yaml to use the latest post processor image

[parthea]

Is your feature request related to a problem? Please describe.
Yes , when synthtool templates are updated, owl bot will automatically open PRs to update the .github/.Owlbot.lock.yaml file with the SHA of the latest post processor image. It is possible for the .github/.Owlbot.lock.yaml update to not be applied if the PR has no other changes as a result of using the latest post processor image. As an example, in this PR, owlbot kept reverting changes because the post processor SHA in .github/.Owlbot.lock.yaml was out of date. If a new PR activates features in the templates which were not previously activated , this can cause confusion, as was the case in this PR. Maintainers need complete the following steps to update the .github/.Owlbot.lock.yaml file:

Pull the latest post processor image locally

 docker pull gcr.io/cloud-devrel-public-resources/owlbot-python:latest

Use this command to get the latest SHA

docker inspect --format='{{.RepoDigests}}' gcr.io/cloud-devrel-public-resources/owlbot-python:latest

Then update the SHA in this file in your pull request : https://github.com/googleapis/python-datacatalog/blob/main/.github/.OwlBot.lock.yaml#L3

Describe the solution you'd like
In an offline discussion @tswast mentioned it would be helpful a label to tell owlbot to automatically update .github/.Owlbot.lock.yaml to use the latest post processor image. Ideally owlbot would push a commit which updates .github/.Owlbot.lock.yaml to avoid this manual step.

Describe alternatives you've considered
An alternative solution is for owlbot to add a comment on a PR which is not using the latest post processor image to let the author know that there is a new version of the post processor image.

Additional context
N/A