[Feature Request] Support Postgres connection via Unix Socket

[langered]

Current Situation:

It is possible to connect a PostgreSQL database via Unix Socket in the SQLAlchemy library:
https://docs.sqlalchemy.org/en/20/dialects/postgresql.html#unix-domain-connections

But it uses a slightly different connection string pattern:

postgres://<user>:<password>@/<database>?host=<db_host>

rag-api is not supporting it because the CONNECTION_STRING is a hard coded pattern which does not support the pattern of the unix socket connection:

rag_api/app/config.py

Lines 71 to 72 in e48968f

	CONNECTION_STRING = f"postgresql+psycopg2://{urllib.parse.quote_plus(POSTGRES_USER)}:{urllib.parse.quote_plus(POSTGRES_PASSWORD)}@{DB_HOST}:{DB_PORT}/{urllib.parse.quote_plus(POSTGRES_DB)}"
	DSN = f"postgresql://{urllib.parse.quote_plus(POSTGRES_USER)}:{urllib.parse.quote_plus(POSTGRES_PASSWORD)}@{DB_HOST}:{DB_PORT}/{urllib.parse.quote_plus(POSTGRES_DB)}"

This would be beneficial when using the Cloud SQL Postgres:
https://cloud.google.com/sql/docs/postgres/connect-run#connect-unix-socket

Proposal:
Adding a feature flag as environment variable, which enables the connection via unix sockets by modifying the connection string.

[langered]

I've opened a PR with my proposed changes:

#153