BE: Implement new query API with filtering support to list available cluster agents under a group in RD

MR: !146453 (merged)

Description

Implement new graphql field remoteDevelopmentClusterAgents under the namespace field as technical design for the new group-agent mapping feature.

This new API must support an optional enum param called filter with the following expected behavior for different enumeration values:

• AVAILABLE: Returns all agents under the group that may be used by a user for workspace creation

An error will be returned if filter is not set in the request. More enum options will be supported in a future issue.

Acceptance Criteria

• Implement new API w/ tests as mentioned in the design doc. These include
• Ensure that the APIs ensure the permissions model as described in the doc
• Ensure that the new APIs/changes to the existing API are behind the feature flag as mentioned in the doc

Technical Requirements

• Same as the acceptance criteria

Design Requirements

NA

Impact Assessment

• There should be no impact as the new API should be behind a feature flag