[C++] ExtensionType subclass for "unknown" types?

In C++, when receiving IPC with extension type metadata for a type that is unknown (the name is not registered), we currently fall back to returning the "raw" storage array. The custom metadata (extension name and metadata) is still available in the Field metadata.

Alternatively, we could also have a generic `ExtensionType` class that can hold such "unknown" extension type (eg `UnknowExtensionType` or `GenericExtensionType`), keeping the extension name and metadata in the Array's type.

This could be a single class where several instances can be created given a storage type, extension name and optionally extension metadata. It would be a way to have an unregistered extension type.

**Reporter**: [Joris Van den Bossche](https://issues.apache.org/jira/browse/ARROW-6179) / @jorisvandenbossche

_{**Note**: *This issue was originally created as [ARROW-6179](https://issues.apache.org/jira/browse/ARROW-6179). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}