[C++] Provide a default implementation for ExtensionType::ExtensionEquals

Check that the extension names and the underlying storage types are equal.

**Reporter**: [Krisztian Szucs](https://issues.apache.org/jira/browse/ARROW-8143) / @kszucs

_{**Note**: *This issue was originally created as [ARROW-8143](https://issues.apache.org/jira/browse/ARROW-8143). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}