[C++] Adding Join Relation ToProto

Adding `ToProto` functionality for Join operator in Substrait. The resultant component is re-creating a `hash_join` operator in ACERO via the received Substrait-relation.

**Reporter**: [Vibhatha Lakmal Abeykoon](https://issues.apache.org/jira/browse/ARROW-16858) / @vibhatha

_{**Note**: *This issue was originally created as [ARROW-16858](https://issues.apache.org/jira/browse/ARROW-16858). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the C++ Join operator and its existing Substrait ToProto conversions, then trace how received Substrait relations recreate an Acero hash_join operator. Confirm the expected relation representation and add coverage showing that a Join round trip produces the corresponding hash_join behavior.

Written by the indexing model from the issue text.