Backend: Add GraphQL mutation to create agent

About

Add a GraphQL mutation for saving agent data.

This will be used by the frontend in #550465 (closed).

It will be possible to save data once #549401 (closed) has been added PostgreSQL tables.

Proposal

🎗️ Remember our frontend-backend collaboration guide and communicate schema ideas on this issue with the frontend counterpart of #550465 (closed).

The mutation must be marked as experimental.

TODO, checking what the intention is of the data we're exposing to better understand how to expose it, and what permissions to use.