Allow for group touch for agents

Current situation

There is no way to 'touch' agents en masse. We have to call the GET /agents/{agent_id} endpoint (or otherwise communicate with the agent channel).

For operators handling large pools, this can be time consuming, and it is inefficient.

Desired outcome

A new PATCH /agents endpoint. The payload is a JSON object, with keys being agent_ids.

PATCH /agents
Content-Type: application/json
Authorization: Bearer xxx

{
  "4b0261cb-9b4b-4110-a97d-938936fa1f74": {},
  "8ff8c9c5-04d9-485d-90e6-484c8a22b21e": {}
}

Requires enough credentials to update the agents resource.