grpc: Add "created_at" to listInvoices rpc

I try to display a nice transaction history, but I cannot insert an unpaid invoice into the timeline at the correct position, as there is no information on when it was created.

Either a "created_at" timestamp is needed, or an "expiry" field has to be provided.
In the second case I could calculate the created_at by doing "expires_at" - "expiry".

Link to the docs for the enpoint that needs the change:
https://docs.corelightning.org/reference/lightning-listinvoices