Resolve NXCALS-4620 "Add missing discretfunctionlist and dflarray types"

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COMPATIBILITY NOTE

Please make sure that this change is BACKWARDS COMPATIBLE and a new server will work correctly with clients using old APIs versions.

It may be that data sent in the new format will break the old clients.

In case of any potential incompatibility we have to inform our clients and schedule release & deployment with them.

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@acc-logging-team - please have a look for approval.

Adding support for DiscreteFunctionList and DiscreteFunctionListArray types. The DiscreteFunctionList type is represented as a record with an array of DiscreteFunctions addressed by a single field "discrete_functions". This is different from our typical multi-array as there exist the type DiscreteFunctionArray (which uses this multi-array representation already).

Closes NXCALS-4620