Add GraphQL fields to check if MR allows mutliple assignees/reviewers

What does this MR do and why?

This adds the following fields to Project type:

• allowsMultipleMergeRequestAssignees
• allowsMultipleMergeRequestReviewers

and the following to MergeRequest type:

• allowsMultipleAssignees
• allowsMultipleReviewers

This can be used by GraphQL consumers to determine whether they need to pass multiple assignees/reviewers to SetAssignees and SetReviewers mutations.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #431829 (closed)