Feat/add-graph-endpoints

[patroza]

Converts some endpoints to a GraphQL like experience:

• Combine multiple queries into a single request. batch queries.
• Optionally request follow-up queries to mutations. so within a single roundtrip we can mutate, and query.
• RPC semantics, so all supported errors are returned within Either Left, instead of encoded to and from HTTP Status Codes + Bodies.

TODOs

• improve type inference; when requesting result or additional follow-up queries, the response type should no longer have these fields optional.
• improve ergonomics, unwrap the Eithers

Future

• select individual fields (concept already done)