[Feature] Add a `graphman database check`

Description

We should add a graphman command that checks that all shards can talk to each other in the way that we need. For each shard, it should access the primary_public (e.g., by running select count(*) from primary_public.chains) and the shard_.._subgraphs.subgraph tables for all the other shards and print any error it encounters.

This would be very useful to ensure that everything is in place for sharding to work as there are many ways in which this can go wrong (firewalls, pg_hba.conf, ..)

Are you aware of any blockers that must be resolved before implementing this feature? If so, which? Link to any relevant GitHub issues.

No response

Some information to help us out

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Beginne damit, die vorhandenen graphman-Datenbankbefehle und den Code zu lokalisieren, der die Shard-Verbindungen verwaltet. Ermittle, wie auf primary_public und die Subgraph-Tabelle jedes Shards zugegriffen wird, und überprüfe anschließend, dass der Befehl jeden Verbindungsfehler meldet und erfolgreich ist, wenn alle Prüfungen bestanden werden.