Fetch initial merge request widget data async

What does this MR do?

This change is behind a feature flag: async_mr_widget

This changes the way the merge request widget is created with data. Currently the widget requires all data to exist in mrWidgetData upon creation - which can cause the page to be slow.

Instead this change allows the widget to be created async. The initial data is requested from the server async and then the widget can render and create itself. gl.mrWidgetData still exists, and only really contains the different paths and routes required inside of the widget.

Closes #31406 (closed)

Screenshots

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

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