Add OAI-PMH reader for use with datastreams

[wgresshoff]

Add an OAI-PMH 2.0 conformant reader for use with datastreams.

# Example config for OAI-PMH reader
{
  "type": "oai-pmh",
  "args": {
    "base_url": "https://gnd.de/oai2d/",
    "metadataPrefix": "oai_dc",
    "set": "authorities",
    "from": "2024-05-23",
    "until": "2024-05-23",
    "verb": "ListRecords",
  }
}

verb is either ListRecords or ListIdentifiers (to get rid of the problem with outdated resumption tokens).