Feature/fresnel merge properties experimental

[kwahlin]

WIP!

• Use FresnelUtil.applyLens instead of JsonLd.toCard for more finer control over what data is indexed.
• Add support for fresnel:mergeProperties, allowing values from multiple fields to be indexed into a single merged field.
• See also Feature/fresnel merge properties definitions#548.

It kind of works together with libris/definitions#548 and a minor frontend tweak but at this stage it's very much a draft. It's untested and the current behavior is different compared to before when using JsonLd.toCard.

TODO:

• Replicate JsonLd.restoreLinks here:

  librisxl/whelk-core/src/main/groovy/whelk/util/FresnelUtil.java

  Line 255 in a99a04f

  //TODO

  (assuming we still want to keep all links)
• Replicate ElasticSearch.recordToChip in FresnelUtil.applyLens. Assuming we still think it's necessary to include adminMetadata for all linked entities.
• Add more search-tokens (see Feature/fresnel merge properties definitions#548) for full _str coverage
• Measure indexing performance
• Add unit tests
• Always search flattened lang fields (e.g. __prefLabel) in search2
• Remove byLang fields from the index and keep only flattened?
• Let _topStr be just _str? See ff67723