add support for OAI-PMH

[peterstadler]

some pointers:

• https://www.openarchives.org/pmh/
• exist-open thread "OAI_PMH packages?"

[peterstadler]

BASE has "Goldene Regeln für Quellenbetreiber" which provide good guidelines.

In general, I believe we'd need to implement a dedicated OAI endpoint which is capable to respond to the 6 request verbs as defined by the standard. For "GetRecord", this could look like e.g. https://weber-gesamtausgabe.de/oai?verb=GetRecord&identifier=A040521&metadataPrefix=oai_dc

[riedde]

Yes, we should extend the basics I tried to develop in the nearer future