Add 'helper' block to connector definition

[skierkowski]

If you look at the current code for connectors, there is A LOT of repeated code. For example, validating parameters, initializing connections, Finding a particular entity, etc. Since the actions and listeners get executed in a different context (in ServiceInstance) instead of the service definition, you can't just define the shared variables/methods in the parent block. A such, one can add a helper block to the service definitions for common methods/variables.