PostgreSQL sink (tracking) #1

Tracking issue for PostgreSQL sink support in GitProxy.

The base sink - a PostgreSQL implementation of the users / repos / pushes / session stores, selectable via the sink config - is proposed in finos#1532. This issue tracks the follow-up features that extend that base into a complete backend. Each is a self-contained PR, stacked on the base, and will be readied for review once the base merges.

Follow-up work

Sub-issues render below automatically.

Base implementation: finos#1532