Enhance merge request widget extension subtext handling

1. Enhance Merge request widget extension to support strings and objects that represent a link + prepended Text as subText inputs.
2. Refactor Code Quality MR Widget to pass in subtext as an object instead of a sprintf string

solves: #354495 (closed)

no visual changes

How to reproduce

Example on how to see this in Action:

1. run Feature.enable(:refactor_mr_widgets_extensions)
2. clone https://gitlab.com/jannik_lehmann/code-quality-test into your gdk
3. open MR in cloned repo including for example this changes jannik_lehmann/code-quality-test!1 (diffs)
4. see codequality MR widget in action