Reorganize spicedb commands to improve ergonomics

[tstirrat15]

Problem Statement

There's an example implementation in #2021, where the concept is that serve and migrate commands are nested under datastore subcommands, such that when you do spicedb postgres serve help, you only see flags that are relevant to that particular use case and datastore. This would further reduce the number of things that a user would have to look through to understand how to invoke and configure SpiceDB for their use case.

Solution Brainstorm

See #2021 as a starting point.