Support SQLite database

[mitchnielsen]

Summary

In #336, we found that the database configuration was centralized within PostgreSQL - meaning database configuration couldn't be provided outside of the postgresql key, which matters if someone wants to use MySQL or even SQLite.

Let's consider providing a new top-level key for configuring the database that's considered if postgresql.enabled=false.

Notes

The Prefect docs mention:

Prefect supports the following databases:

1. SQLite (default in Prefect): Recommended for lightweight, single-server deployments. SQLite requires essentially no setup.
2. PostgreSQL: Best for connecting to external databases, but requires additional setup (such as Docker). Prefect uses the pg_trgm extension, so it must be installed and enabled.

Elsewhere, it says:

Use SQLite database for testing only
SQLite does not support multiprocessing. For high orchestration volume, use PostgreSQL.

So realisticically, most users should be on PostgreSQL. But the documentation shows you how to use SQLite, so I think it's reasonable to support that configuration in the charts. As mentioned in #365, users are already using SQLite with the charts with some workarounds.

Acceptance criteria

• Database configuration can be provided outside of the top-level postgresql key if PostgreSQL is not in use.
• SQLite specifically is tested to confirm nothing is hard-coded to work with PostgreSQL

[mitchnielsen]

I bet this was indirectly addressed by https://github.com/PrefectHQ/prefect-helm/pull/365, but let's try it to be sure and add an SQLite test case to be sure.