Add modal support to widget framework

What does this MR do and why?

Related: #358187 (closed), #357523 (closed)

This MR adds modal support to the merge request report widget framework, which ~"group::pipeline insights" needs for the test summary widget and groupthreat insights needs for the security reports widget.

Screenshots or screen recordings

Screen_Recording_2022-04-08_at_16.38.42

How to set up and validate locally

See !84641 (merged) and its testing instructions for an example widget (shown above)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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