Support OR filters in IssuableFinder [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Allows OR filtering in issues / MRs for author, assignee, label, milestone, release, and my reaction.

#296031 (closed)

This does not affect Epics yet because EpicsFinder overrides #execute. This also does not include issue-specific / MR-specific fields like weight, epic, etc..

Testing

1. Enable the or_issuable_queries feature flag.

2. Sample URLs:

   • http://localhost:3000/h5bp/html5-boilerplate/-/issues?or[author_username][]=root&or[author_username][]=dann_lakin
   • http://localhost:3000/h5bp/html5-boilerplate/-/issues?or[label_name][]=label1&or[label_name][]=label2

(There is no frontend for this right now)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team