Support for beta feature Concurrency

[jlprat]

GitHub workflows have now a feature in beta, concurrency. One can read more in here: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency

The feature enables to stop an already running workflow when a new one is enqueued. Enabling this would save lots of time on CI servers for jobs that are already obsolete.
To configure it, this should be added to the continuous integration yml file:

concurrency:
  group: ci-${{ github.ref }}
  cancel-in-progress: true

@djspiewak Would you consider adding this feature even though is still in Beta on GitHub side? I would be willing to contribute it.

[djspiewak]

@jlprat Finally got around to this! This looks really awesome honestly, and feels like something that should definitely be well integrated into this plugin. Would love to have it as a feature!

In particular, I think we should ensure that it's configurable, but the default should be to group based on Pull Request specifier (I believe that is what github.ref covers) or upstream branch if not on a PR.

[pjfanning]

@mdedetrich this is useful. sbt-typelevel's equivalent plugin to this supports it.

[mdedetrich]

There are plans to upstream this