Allow LeafFieldComparators to publish feature values [LUCENE-8949]

We allow LeafFieldComparators to only accept a docID, get the equivalent feature value(s) and compare against the bottom/top of the values set for the comparator. This mandates that the values being compared against the bottom/top should originate from the same comparator. This does not allow use cases such as cross comparator value comparisons i.e. if a user wanted to compute the "global" minimum across multiple comparators.

FieldComparators expose an API to get the feature value corresponding to a docID. We should let LeafFieldComparators do the same. A new comparison method is not required since the parent FieldComparator's compare method can be used once the values are retrieved.

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Migrated from [LUCENE-8949](https://issues.apache.org/jira/browse/LUCENE-8949) by Atri Sharma (@atris), updated Sep 26 2019

Start by reading the FieldComparator and LeafFieldComparator APIs described in the issue, focusing on how a docID is currently converted into a feature value and compared with the bottom or top value. Done means LeafFieldComparators can expose the value for a docID so callers can use the parent FieldComparator comparison for values from different comparators.

Written by the indexing model from the issue text.