Support running dstack server with Postgres

[r4victor]

Currently, dstack supports only SQLite as the server's database. SQLite works great when running the server locally but may not be suitable for production deployments. The proposal is to let users opt-in to use Postgres.

SQLite also imposes a restriction that the dstack server cannot have multiple replicas. We can lift this restriction when using Postgres. This will require changing how concurrent access is handled and is to be addressed in a different issue.