WIP: Adding initial implementation of feature to enable setting default mr target branch per project

What does this MR do?

This MR adds a feature to enable users to set a default merge target branch on a per project basis. This is useful for some development workflows where the default_branch is less regularly the desired target. see #17909 (closed)

Todo

• Create a new field on the project model
• Tie the field in with graphql
• Add a front-end component to project MR settings
• Create a method for updating the new field
• Link the new field to the front-end

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