OperationMetadata without default options

[tomaszmichalak]

Is your feature request related to a problem? Please describe.
OperationMetadata (a part of NodeMetadata) contains details about action's configuration options (ActionNodeConfig). We use the syntax:

factory = action
config { 
   # the value below is not set, it comes from POJO
   # someKeyWhichValueIsValid = a 
   keyWhichValueIsOverriden = b
}

In the config node, we specify some custom settings. Now, OperationMetadata contains custom settings only.

Describe the solution you'd like
We expect to have both custom and default settings in OperationMetadata.

Additional context
This is required by Knotx/knotx-fragments-chrome-extension#98.

[marcinus]

Please refrain from changing contracts for the main domain objects like ActionFactory.
The top-level abstractions should only change when major version number is updated and should be designed with care.
If changing ActionFactory contract is absolutely necessary, please provide default methods in interfaces to preserve backward compatibility.

[tomaszmichalak]

To verify.