Introduce ConfigExtensionExt, allowing the propagation of arbitrary ConfigExtensions across network boundaries (#100)

* Add `ConfigExtensionExt`, allowing the propagation of arbitrary [ConfigExtension]s across network boundaries

* Rename propagate_distributed_option_extension to retrieve_distributed_option_extension

* Change x-datafusion-distributed- to x-datafusion-distributed-config-

* Check for full format!("{}.", T::PREFIX) in gRPC keys

* Remove double clone

* Fix tests

* Fix tests

Author: gabotechs