Add SRU opener / open-sru

[TobiasNx]

Like OAI PMH opener, metafacture should be able to retrieve all records of a SRU request.
A new modul should be configurable with the specific SRU keywords.

The opener should be similarl to gist.github.com/jorol/989ea5dd464fb85bd99b4733710d2890#oai-pmh and https://
github.com/LibreCat/Catmandu-SRU/tree/dev

In Flux something like:

| open-sru(base="https://services.dnb.de/sru/zdb", recordSchema="MARC21-xml", query="tit = soil biology", parser="marcxml")

We have two use-cases:

• Upcoming introduction to metafacture in Potsdam they have knowledge about SRU so it would be handy to show them something they know.
• In context of RPB we need to fetch data from HeBis via SRU: RPB-225

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Possible other Java sources that could help develop an SRU opener for Metafacture:
https://github.com/opacapp/opacclient/blob/master/opacclient/libopac/src/main/java/de/geeksfactory/opacclient/apis/SRU.java
https://github.com/rism-international/sru-downloader
https://github.com/indexdata/cql-java
https://www.loc.gov/standards/sru/resources/products.html

[TobiasNx]

I cleaned up to comments to make this issue more compact.