Update Add Members Api to accept user_id list

What does this MR do?

Updates Add Member API post :id/members to accept a list of IDs for user_id. Previously only accepted one.

Related to !42398 (merged)

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/217922

Screenshots

Using user_id for multiple IDs	Response:

user_id for single id (old behavior)	Responses:
	Success: Error:

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

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