Ablity to specify preshared_key and datastore_uri in different secrets and with custom keys

[tokongs]

If you don't completely control how the datastore_uri is generated(and possibly regenerated at any time), such as with a Postgres operator, creating a secret for the SpiceDBCluster config is hard.

The ability to specify these parameters in different secrets and with custom keys inside the secrets would be great!

[ecordell]

Thanks for the suggestion; I agree that this should be more flexible. There is also some overlap with #155