Provide custom rules to add new rules

[mariusfilipowski]

Provide a possibilty to add custom, optional rules. We would like to check for the naming, ids and also the runner label combination in our context and provide this as custom rules that are run on top of the standard rules of action lint.

Is this possible?

[hugo-syn]

Yes check this PR https://github.com/rhysd/actionlint/pull/327