Harvester for InGrid via OpenSearch for UVP-Portal

The "UVP-Verbund der Länder" is backed by an InGrid installation and exposes an OpenSearch-compatible (the XML-based API specification, not Amazon's ElasticSearch fork) API using a custom schema for the items. This API filtered for Saxony-Anhalt as an example is reachable via https://www.uvp-verbund.de/interface-opensearch/query?q=partner:st&ingrid=1&detail=1 which further documentation on the underlying InGrid module available here.

Acceptance criteria:

• Harvester for InGrid via OpenSearch merged into main branch and deployed to md.umwelt.info.
• One dataset for each item and one resource for each link.
• Reasonable coverage for the available metadata, either via schema changes or via follow-up stories for schema changes.