For runtime fields, we will want to do all search-time interaction with
a field definition via a MappedFieldType, rather than a FieldMapper, to
avoid interfering with the logic of document parsing. Currently, fetching
values for runtime scripts and for building top hits responses need to
call a method on FieldMapper. This commit moves this method to
MappedFieldType, incidentally simplifying the current call sites and freeing
us up to implement runtime fields as pure MappedFieldType objects.
01950bc80f