Allow setting of OperatorRef in Siri-SX subscription request

[leonardehrenfried]

For a Siri-SX subscription request I needed to set the <OperatorRef> field.

I found an example of how to do it in the Siri-ET code and tried to convert it to the Siri-SX one.

However, I'm not sure if I fully understood how these filterMaps are supposed to be used.

@lassetyr Can you check the config example if this is correct?

[lassetyr]

The original idea was that the subscription-config should use an enum-value from SubscriptionPreset, and that this enum would be processed in FilterMapPresets.

Originally, this was done to support defining a list of filter-values - e.g. several lines like this case.

I have not tested the code in this PR, but it looks like it works as expected, and it also seems like a more generic approach that will support most usecases.

Could you also create an example for how this could be used to define a list of values in the subscriptions.yml-file?

[leonardehrenfried]

I would need to adjust the parsing code but I think it could work something like this:

filterMap:
  uk.org.siri.siri21.OperatorRefStructure: 
    - "AN_OPERATOR"
    - "ANOTHER_OPERATOR"

Do think that this kind of mechanism is a good idea or do we want something more workflow-specific like:

operatorRef:
  - "AN_OPERATOR"
  - "ANOTHER_OPERATOR"