Add Danger check for empty introduced_by_url fields

What does this MR do and why?

I often forget to fill introduced_by_url field while adding a new FF or event/metrics definition.

So I created this Danger check to figure out such fields and suggest url based on MR.

Screenshots or screen recordings

I did commit some config files to test how the plugin works:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.