Create automated check for identity breakage

[cloutiertyler]

In the 0.10 release, we were surprised by user identities being invalidated. I believe this was due to some paths changing, so the servers' keys for signing the identities could not be found.

This will not be acceptable post-1.0, so we should have some better way of noticing that identities are going to be invalidated.

Definition of Done

Not sure what the exact right shape for this is! Whoever takes this ticket should probably propose something specific.

[cloutiertyler]

The DOD of this ticket should be a test to ensure that the identity signing secret directory doesn't change. This is also related to: https://github.com/clockworklabs/SpacetimeDBPrivate/pull/850