[C++] Adding Fetch Node ToProto

For roundtrip testing adding the Fetch declaration serialization. 

**Reporter**: [Vibhatha Lakmal Abeykoon](https://issues.apache.org/jira/browse/ARROW-17504) / @vibhatha

_{**Note**: *This issue was originally created as [ARROW-17504](https://issues.apache.org/jira/browse/ARROW-17504). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by locating the Fetch declaration and the existing ToProto serialization entry point in the C++ code. Review the roundtrip testing path and related node serialization before making changes. Done means Fetch declarations serialize successfully and roundtrip testing verifies the result.

Written by the indexing model from the issue text.