Extract and move Secret Detection Vuex module to CE

What does this MR do?

Extract and move Secret Detection module to CE

This extracts and moves most of a Secret Detection 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 Secret Detection 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.

A similar approach was taken for the SAST module.

This MR is made of a few commits, and it may be worth reviewing them separately, since they contain relatively distinct stages of the work.

Other changes include:

• Extracting constants, as per !47067 (comment 445054667)
• Fixing a minor bug regarding real-time dismissal updates
• Aligning the new Secret Detection module to be closer to the SAST module, easing future refactoring work to deduplicate them
• Using the secretDetection nomenclature more than secretScanning (though not everything is conforming - see relevant commit message)

Screenshots (strongly suggested)

There's no visual change, except for the fix included which makes real-time updates of the dismissal status of vulnerabilities work for secret detection findings:

Before	After
master_dismiss_secret_vuln	branch_dismiss_secret_vuln

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)