feat: return information on attached agents when getting list of sources from API

[cpacker]

GET sources now returns extra metadata:

{
  "attached_agents": [
    {"id": "XXX", "name": "YYY"},
    {"id": "XXX", "name": "YYY"},
  ]
}

,
		{
			"name": "custom-source3",
			"description": null,
			"user_id": "00000000-0000-0000-0000-000000000000",
			"created_at": "2024-03-18T16:33:56.517006",
			"id": "374a1aea-543f-4103-82e4-39ab24812209",
			"embedding_config": {
				"embedding_endpoint_type": "openai",
				"embedding_endpoint": "https://api.openai.com/v1",
				"embedding_model": "text-embedding-ada-002",
				"embedding_dim": 1536,
				"embedding_chunk_size": 300
			},
			"metadata_": {
				"num_documents": 0,
				"num_passages": 0,
				"attached_agents": []
			}
		}
	]