Adding pgsql datasource for Grafana

[marcleblanc2]

For PS-83: Create pgsql datasource in Grafana

Checklist

• Sister deploy-sourcegraph change:
• Sister customer-replica change (if necessary, for any changes affecting pure-docker or configuration):
• All images have a valid tag and SHA256 sum

Test plan

Tested on test instance, with and without env vars configured

Alternatives Considered

Managing the postgres datasource as a separate file in the datasources directory is preferred over adding it as another datasource in the existing file, and as Docker Compose mounts an entire directory to a directory, this works just fine.