Gitlab Community Edition Instance

Added a way to request additional information from a search backend.

# Conflicts:
#	apidoc/src/api.yaml
#	cdstar-plugins-pom/cdstar-proxy-search/src/test/java/de/gwdg/cdstar/ext/proxysearch/ProxySearchProviderTest.java
#	cdstar-rest/src/main/java/de/gwdg/cdstar/rest/v3/async/SearchHandler.java
#	cdstar-web-common/src/main/java/de/gwdg/cdstar/web/common/model/SearchHits.java
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