feat: return source metadata with list sources route

[cpacker]

GET http://localhost:8283/api/sources now returns metadata along with source information.

NOTE @sarahwooders : embedding_config and description fields are currently bugged until Source is deprecated in favor of pydantic SourceModel throughout the code (punting to separate PR).

GET http://localhost:8283/api/sources

{
	"sources": [
		{
			"name": "custom-source",
			"description": null,
			"user_id": "00000000-0000-0000-0000-000000000000",
			"created_at": "2024-03-17T16:48:54.418499",
			"id": "01e0035f-c506-4f3b-adfc-6ed920725de2",
			"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
			},
			"source_metadata": {
				"num_documents": 0,
				"num_passages": 0
			}
		},

Have you tested this PR?

- [ ] Replace num_documents / num_passages placeholder values with real data

• -> future PR

[sarahwooders]

LGTM!

list_data_source_passages should be paginated, but we can fix that when it gets actually implemented.