Validate used action's `with:` clause

[TWiStErRob]

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Following validation of existence of an action and tag, I think it would be awesome if the validator grabbed action.yml and validated the with: block for required values. This would allow us to detect typos or mis-configurations early, especially in workflow_call and similar situations. Similarly for workflow_call situations, the inputs: could be checked for required and type inconsistencies in the calling with: block.

Additionally there could be a warning emitted if the default matches a provided value, but this is probably controversial, so might be worth treating it as a separate issue/enhancement.

cc @award28 FYI

[mpalmer]

I wouldn't say no to a PR implementing that.