feat: add agent service to check availability

[anvh2]

🧠 Rationale behind the change
In our production environment, we need to manage agents registered with LivekitServer. Through load testing, we can determine how many jobs a single agent worker can handle. Therefore, before dispatching an agent to a room, we want to check the availability of the agent.
However, the agent state is managed per LivekitServer instance, and we cannot retrieve all agent workers from the cluster. To address this, we plan to poll the agent state from all nodes and cache that state with a TTL in our dispatch service.

[changeset-bot]

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