feat(substrait): introduce consume_rel and consume_expression (#13963)

* feat(substrait): introduce consume_rel and consume_expression

Route calls to from_substrait_rel and from_substrait_rex through the
SubstraitConsumer in order to allow users to provide their own behaviour

* feat(substrait): consume nulls of user-defined types

* docs(substrait): consume_rel and consume_expression docstrings

Author: vbarua