UnifiedHighlighter: Use a different stored field, possibly shared [LUCENE-7768]

UnifiedHighlighter uses stored field content to highlight. It has some disadvantages, because index grows up fast when using multilingual indexing due to several fields has to be stored with same content.

Lucene portion of issue [SOLR-1105](https://issues.apache.org/jira/browse/SOLR-1105), initially raised on Solr
See https://issues.apache.org/jira/browse/SOLR-1105

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Migrated from [LUCENE-7768](https://issues.apache.org/jira/browse/LUCENE-7768) by Julien Martin (@Julm), 1 vote
Linked issues:
- [SOLR-1105](https://issues.apache.org/jira/browse/SOLR-1105)

Start by reading the UnifiedHighlighter behavior described in this issue and the linked SOLR-1105 and LUCENE-7768 discussions to determine the intended shared stored-field design. Identify the affected highlighter entry points and tests in the Lucene repository; done means multilingual highlighting no longer requires duplicating the same content across several stored fields while existing highlighting behavior remains covered.

Written by the indexing model from the issue text.