SOLR-12457: improve compatibility/support for sort by field-function

[magibney]

See: SOLR-12457 Continuation of: https://github.com/apache/lucene-solr/pull/989

Affects marshal/unmarshal of sort values for field-function sorts, native numeric wrapper for multivalued Trie field docValues, and missingValue (sortMissingFirst/sortMissingLast) wrt field-function sort.

[magibney]

Cleaned this up quite a bit.

The essence of what's happening here is pretty straightforward:

1. ValueSources need to supply their own SortField, but in Solr they need to supply a SortField that is aware of missing values, etc., so we wrap Lucene ValueSource impls to delegate getSortField() to the Solr FieldType.
2. We leverage FieldCacheSource (which I finally now think I understand its purpose) to determine when/how to do specialized marshalling/unmarshalling of sort values.

That's it!

I plan (pending feedback/approval) to add some javadocs to Lucene FieldCacheSource clarifying its uses, and change Lucene JoinDocFreqValueSource so that it no longer extends FieldCacheSource (I think it never should have). Assuming that goes as I anticipate, we'll be able to remove the special-case for JoinDocFreqValueSource in SortSpecParsing. If it doesn't go as I anticipate, will re-evaluate and decide how to proceed.

[janhoy]

Hi @magibney . I got a question about this bug from a client. Do you remember why this was not merged back in May'22?

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