PostgreSQL sink: connection configuration (split env vars, pool & TLS options)

[dcoric]

Follow-up to #1497 / #1532.

Only a single connectionString (or GIT_PROXY_POSTGRES_CONNECTION_STRING) is supported, and the pool is created with all defaults (new Pool({ connectionString })).

Scope

• Support split PGHOST / PGPORT / PGUSER / PGPASSWORD / PGDATABASE.
• Expose pool sizing / timeout configuration.
• Provide structured TLS/ssl options (parity with the mongo backend's options object), instead of requiring sslmode to be embedded in the connection string.

[dcoric]

Delivered by #1584, merged into the feat/postgres integration branch; lands on main with #1687.