facilitate date-related FieldValueFeature extensions (docValues=True only)

Currently having custom features such as relative "document age" or "remaining shelf life" based on a date field requires duplication of a lot of the FieldValueFeature code:

• https://github.com/apache/solr/blob/releases/solr/9.3.0/solr/modules/ltr/src/java/org/apache/solr/ltr/feature/FieldValueFeature.java

The NumericDocValuesFieldValueFeatureScorer.readNumericDocValues() implementation already considers the float and double NumberType choices and by also considering NumberType.DATE there we could facilitate date-related extensions:

• https://github.com/apache/solr/blob/releases/solr/9.3.0/solr/modules/ltr/src/java/org/apache/solr/ltr/feature/FieldValueFeature.java#L251
• https://github.com/apache/solr/blob/releases/solr/9.3.0/solr/core/src/java/org/apache/solr/schema/NumberType.java#L27