implement pg query for retrieving keys

[rsdoiel]

The OAI-PMH API is too slow to use for key retrieval. It should only be a fallback. I need to add support for querying Postgres directly for record ids (GetRecordsIds and GetModifiedRecordIds). Since irdmtools maybe working with both MySQL (EPrints) and Postgres (RDM) I need to add explicit DB variables to setup connects.

For rdmutil it can check if DB connection is define then directly query Postgres for the ids.

[rsdoiel]

Implemented Postgres query option for GetRecordIds, will need to sort out what fields I need to use to create the last id list for modified/created records in a time range.

Query for all records ids

SELECT COUNT(json->>'id')  FROM rdm_records_metadata WHERE json->'access'->>'record' = 'public';
``

I'm assuming I should never return restricted records. I could add an option for that and adjust the query accordingly.

[rsdoiel]

This will be available in the upcoming v0.0.55 release. Still evaluating if using the REST API for JSON object retrieve is fast enough. Most of what we need is in the rdm_records_metadata.json column, there is some additional object construction required for .files, .parents and maybe .pids.