Add /depend_on quick command for merge requests

What does this MR do?

This MR introduces a new /depend_on quick command that allows users to easily set one merge request as being dependent on another before it can be merged.

The command leverages the existing blocking merge requests infrastructure available in GitLab EE.

How does it work?

Users can now use the /depend_on quick action in merge request comments to set dependencies:

/depend_on !123

Or specify multiple blocking merge requests:

/depend_on !123 !456

Implementation details

• New module: EE::Gitlab::QuickActions::MergeRequestDependencyActions
• Integration: Added to EE::QuickActions::InterpretService
• Documentation: Updated merge request dependencies documentation to include quick action usage
• Comprehensive specs: Full test coverage for the command including edge cases

Benefits

• Faster workflow for setting merge request dependencies
• Consistent with other quick actions in GitLab
• No need to navigate to the edit page to set dependencies

Related documentation

• https://docs.gitlab.com/ee/user/project/merge_requests/dependencies.html

Changelog: added EE: true