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

[n1ru4l]

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.