Add PyDough support for PostgreSQL dialect #384
Description
What is required:
- Adding new Database enums & connector support.
- Updating any bindings (if needed) to work for MySQL/SingleStore functions.
- Ensuring there are comprehensive SQL generation unit tests for the dialect that cover the functions.
- Adding test infrastructure that can spin up / tear down a PostgreSQL server (both locally for pytest & in CI), along with the same conditional running as described in Add support for Snowflake databases & dialect in PyDough #296
- User documentation for how to use PyDough with PostgreSQL