feat: add shared workflow

[rgychiu]

This adds a called workflow to allow other GHC repositories to use this instead of having to rewrite the CLA bot workflow from scratch. In addition, this allows for updating the workflow without needing to update the downstream repositories.

Notes

• The reusable workflow copies all inputs from the action. They typically aren't used and are defaulted, but lets the workflow be more configurable.
• Required checks should be updated since the reusable workflow changes the check name
• Follow up to this PR requires a batch change to update all repos with dependencies on this workflow.

[github-actions[bot]]

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[rgychiu]

Note to self to change the workflow reference to @main instead of a specific hash before merging. This was changed to be able to run the workflow to test without needing it to be merged.

[rgychiu]

Can we instead just have clabot.yml have the same check name though? Why do we need to update the downstream repositories?

Did some digging and it seems like the check name changes since we're using a reusable workflow. Also couldn't find anything to see if I could keep the check name the same, but I could be missing something.

We have to update the downstream repositories since they all have their own local copy of the cla bot workflow. They need to be updated to v2.0.2 since I released that version, but we can change their workflows to leverage this shared workflow instead. This lets us update the bot version in this repository and not have to touch the other repos after the batch change.