BE: Migrate existing agent resolver logic to dedicated field maintained by RD team
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MR: Pending
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## Description
This is created as a follow-up to the discussion in this [thread](https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/merge_requests/44#note_1752809517).
For some background, as a part of supporting group-agent authorization strategy, a decision was made to create a new field `remoteDevelopmentClusterAgents` under `group` query to list all the cluster agents with remote development support (with configurable filters).
```graphql
group(fullPath: "gitlab-qa-sandbox-group-6") {
id
remoteDevelopmentClusterAgents(filter: DIRECTLY_MAPPED) {
nodes {
id
name
}
}
}
```
As a part of this discussion, another conclusion was reached to move the existing logic under the following graphql query resolver to the resolver for this new field. Since this new field is maintained by RD team, this migration would enabled greater control over this part of domain logic, as the current query resolver that returns all agents under a group is not maintained by RD team.
```graphql
group(fullPath: $groupPath) {
id
clusterAgents(hasRemoteDevelopmentEnabled: true) {
nodes {
id
name
project {
id
nameWithNamespace
}
}
}
}
```
## Acceptance Criteria
- [ ] Existing dependency on `group.clusterAgents(hasRemoteDevelopmentEnabled: true)` in Remote Development code is replaced with with calls to `group.remoteDevelopmentClusterAgents`. Functionally speaking, `group.remoteDevelopmentClusterAgents` should have the contract as `group.clusterAgents(hasRemoteDevelopmentEnabled: true)`
- [ ] Add specs to verify this change
- [ ] Ensure all code/references/docs related to prior resolver logic in Cluster Agent domain is removed/deleted.
## Technical Requirements
Same as acceptance criteria
## Design Requirements
NA
## Impact Assessment
NA
## User Story
NA
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