Add an adapter for oersi.org as source option

[JannikStreek]

Description

As the available content currently is very limited, additional sources are important. One more source that provides an open API is oersi.org.

Possible query:

curl -X 'POST' 'https://oersi.org/api/search/oer_data/_search?pretty' \
  -H 'Content-Type: application/json' -H 'accept: application/json' --user-agent 'MyBot/1.0 (https://example.org/mybot/; mybot@example.org)' \
  -d '
{
  "size": 20,
  "from": 0,
  "query": {
    "bool": {
      "must": {
        "multi_match": {
          "query": "Mond",
          "fields": ["name", "description", "keywords"]
        }
      },
      "filter": {
        "match": {
          "encoding.encodingFormat": "image/png"
        }
      }
    }
  },
  "sort": [{"id": "asc"}]
}'