Add mutation APIs for group-agent authorization strategy

Issue: #440545 (closed)

What does this MR do and why?

• Add Create/Destroy APIs for group agent authorization strategy
• Add specs to verify the new APIs introduced
• Add feature flag that controls access to the new APIs

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Examples of mutations introduced

mutation createGroupAgentMapping {
  namespaceCreateRemoteDevelopmentClusterAgentMapping(input:{
    clusterAgentId: "gid://gitlab/Clusters::Agent/8"
    namespaceId: "gid://gitlab/Group/74"
  }) {
    errors
  }
}

mutation destroyGroupAgentMapping {
  namespaceDestroyRemoteDevelopmentClusterAgentMapping(input:{
    clusterAgentId: "gid://gitlab/Clusters::Agent/8"
    namespaceId: "gid://gitlab/Group/74"
  }) {
    errors
  }
}

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.