Add feature: add api for resolve conflict

What does this MR do?

Gitlab does not provide an interface part that resolves merge request conflicts online. We have added the interface of this part according to our own business needs. There are three interfaces, which is convenient for users to use the interface to complete the online resolution of the merge request conflict when not using the gitlab native page.

What are the relevant issue numbers?

It's a new feature, so there are no relevant issue numbers.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides