SysBackendMethods Need (Slight) Refactoring

[clauspruefer]

Problem

SYSBackendMethods handling inconsistent for multiple use-cases.

Description

SYSBackendMethods handling sub-optimal, especially when multiple backend methods are neccessary. Also no SYSBackendMethod can be specified in service call data (currently only in the backend).

Requirements

• A backend method which will be executed must be specified in the service call metadata, not only in the backend
• A backend method must be assignable to a class

[clauspruefer]

There is no need to specify backend methods in the way it is currently done. See in the next implementation proposal / PR.