[C++] Add RoundTrip to Relations

[asfimport]

In the effort solving https://github.com/apache/arrow/issues/31862, the tasks have been structured into a set of tasks. The focus of this task is to provide the ToProto function for relations in Substrait. This task will include a set of child tasks which include a set of relations added in each child task. 

Reporter: Vibhatha Lakmal Abeykoon / @vibhatha

Subtasks:

• [C++] Adding Read Relation ToProto
• [C++] Adding Filter Relation ToProto
• [C++] Adding Project Relation ToProto
• [C++] Adding Join Relation ToProto
• [C++] Adding Aggregate Relation ToProto
• [C++] Adding Union Relation ToProto

Related issues:

• [C++] Add roundtrip support to plans + relations (is depended upon by)
• [C++] Add Roundtrip Support to Plans (is depended upon by)

_{Note: This issue was originally created as ARROW-16854. Please see the migration documentation for further details.}

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned per project policy. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.