Add a functional 'allow-failure' in Actions

[MSP-Greg]

For quite a while people have asked for a functional 'allow-failure' options in Actions. Posts exist in the community forum, and issues exist in other repos.

See https://github.com/actions/runner/issues/2347 and the comments that follow. At present, the implementation is somewhat inconsistent, as one web page (the Actions run list) can show an event as passed, but the event's pass/fail indicator shows a failure.

I don't know where this issue should be filed, as it is related to the web UI. I've never needed to use the API (rest or GraphQL) to retrieve pass/fail data from either 'events ' or 'runs'.