BE: Implement the create/destory mutation APIs for group-agent mappings for RD as new agent exposure strategy

MR: !145387 (merged)

Description

Implement the create/destroy mutation APIs behind the feature flag as described in the technical design for the new group-agent mapping feature

Acceptance Criteria

• Implement the CRUD APIs w/ tests as mentioned in the design doc. These include
  • API to create new group-agent mappings
  • API to delete new group-agent mappings
• Ensure that the APIs ensure the permissions model as described in the doc
• Ensure that the new APIs are behind the feature flag as mentioned in the doc
• Ensure that a descriptive and standard error is returned if attempting to add a mapping which already exists.
• Ensure that a descriptive and standard error is returned if attempting to remove a mapping which does not exist.

Technical Requirements

• Same as the acceptance criteria

Design Requirements

NA

Impact Assessment

• There should be no impact as all of the new APIs should be behind a feature flag