Add REST API to manage Cluster Agents
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### Release notes
GitLab now includes a REST API to manage Agents and Agent Tokens. This supplements the existing GraphQL API.
### Problem to solve
Many community tools rely on GitLab's REST API. With only a GraphQL API to manage agents, these tools are not able to manage agents and agent tokens.
### Proposal
Add a rest API to manage `Cluster::Agent` and `Cluster::AgentToken`
The proposed API endpoints are described in [the OpenAPI proposal MR](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75506) and [can be viewed here](https://gitlab.com/gitlab-org/gitlab/-/blob/345603-add-rest-api-to-manage-cluster-agents/doc/api/openapi/agent.openapi.yaml)
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