codeintel: RFC 235: Add postgres-backed store implementation

Created by: efritz

A step towards RFC 235: Move LSIF data to Postgres. This a prerequisite to https://github.com/sourcegraph/sourcegraph/issues/13886, https://github.com/sourcegraph/sourcegraph/issues/13888, and https://github.com/sourcegraph/sourcegraph/issues/13889.

This PR adds a bundle store implementation that targets Postgres, but does not update any place where SQLite is currently used. These need to be added in a certain order to ensure we do not write data to a place we're not querying. Do not use this new code until https://github.com/sourcegraph/sourcegraph/issues/13887 has been completed and deployed.