Add string/double Fluent Dictionary Option to MultiMatchQuery

* For the MultiMatchQuery, this adds an override to OnFieldsWithBoost to
allow one to specify a list of fields and boost values with string
values instead of fluent values.
** This adds parity to QueryStringQuery.OnFieldsWithBoost

This is useful in the case of multifields where you have no fluent
equivalent of the field name you want to query and boost on.