Add Infrastructure Registry sidebar entry [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds a new entry to the Packages & Registries sidebar: Infrastructure Registry This work is behind feature flag, please see the plan for a full overview

The Plan

Description	MR
Add sidebar and pages + feature flag	👈 you are here
Modify package list app to be customizable	!57829 (merged)
Wire and customize package list app for terraform	TBD
Modify package details app to be customizable	TBD
Wire and customize package details app for terraform	TBD
Final UX check, documentation and feature flag removal	TBD

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 there is a development feature flag.
• [-] 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

Related to #323418 (closed)