Function get_protocol_handler 

async fn get_protocol_handler(
    __arg0: AuthenticatedClientRequest<Request>,
) -> Result<ArminiusResponse<Response>, ThirdPartyError>

Handler for GET /_matrix/client/v3/thirdparty/protocol/{protocol} (See https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv3thirdpartyprotocolprotocol).

Fetches the metadata from the homeserver about a particular third-party protocol.

NOTE: For now, there are no supported third-parties, so this always returns 404 - Not Found.