This converts RankFeatureFieldMapper, RankFeaturesFieldMapper,
SearchAsYouTypeFieldMapper and TokenCountFieldMapper to
parametrized forms. It also adds a TextParams utility class to core
containing functions that help declare text parameters - mainly shared
between SearchAsYouTypeFieldMapper and KeywordFieldMapper at
the moment, but it will come in handy when we convert TextFieldMapper
and friends.
Relates to #62988