Add glfilteredsearch to epic boards [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

After !54641 (merged) was merged we decided to change gears and start the gl-filtered search refactor to Epic Boards

Repro

1. rails c
2. Feature.enable(:epic_boards) && Feature.enable(:boards_filtered_search)
3. Go to Groups -> Epics -> Boards

What to look for

Whats included in this MR

• Just the GlFilteredSearch Skeleton

Whats not included in this MR

• Search functionality
• Tokens
• Recent search

Whats Next

• Token
• Search (Blocked by backend currently)
• Token Search (Blocked by backend currently)
• Moving epic info to a module

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• 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