Function get_user_for_protocol_handler 

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

Handler for GET /_matrix/client/v3/thirdparty/user/{protocol} (See https://spec.matrix.org/v1.15/client-server-api/#get_matrixclientv3thirdpartyuserprotocol).

Retrieve a Matrix User ID linked to a user on the third-party service/.

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