Move SAST Vuex module to CE [RUN AS-IF-FOSS]

What does this MR do?

Move SAST Vuex module to CE

This moves (most of) the SAST Vuex module from EE to CE as part of the effort to add vulnerability counts to the CE MR security widget.

It's not a straight-forward move, though. The SAST module contains actions, mutations and getters specific to EE functionality, so the approach taken here was to move all common behaviour to CE version, and make the EE version extend the CE version.

Screenshots (strongly suggested)

n/a

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #273423 (closed)