expose subgraph introduction date for a target/graph via the GraphQL API

We currently have no way of providing information on when a subgraph was first published/added to a supergraph.

We do not store this information, but could compute it based on existing postgres database state, however, the query would not be straight-forward.

For newer projects it would make sense to actively track it in new database tables, to reduce computation overhead.

Start by reviewing the GraphQL API entry points and the existing PostgreSQL database state used for subgraph and supergraph data. Determine whether the introduction date should be computed from existing records or tracked in new database tables, then expose that date through the target/graph API and verify the result for newer projects.

Written by the indexing model from the issue text.