Add a new Confluence integration service

What does this MR do?

This MR adds a new ConfluenceService integration, which, when enabled provides a new Confluence side nav item that links to a Confluence Cloud URI, replacing the regular GitLab Wiki nav item.

• Issue: #220934 (closed)

Feature is behind the :confluence_integration feature flag.

Screenshots

A demo of the feature can be viewed here: https://drive.google.com/file/d/1DT-cyx6Q6j0FLslMqi7dnxomlId4y_sI/view?usp=sharing - note that the changes to the empty state of the wiki in the demo will be in a separate (future) MR.

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