Sending a data element groups with an arbitrary number of fields

[raphaelm]

For HKCAZ, we are expected to supply a list of supported URNs. However, as far as I can see, our TypedField mechanism doesn't allow such a thing, since for every DataElementGroupField, we need to supply the child type which can't be ValueList at the moment.

This is not yet really relevant because I could only find urn:iso:std:iso:20022:tech:xsd:camt.052.001.02 in the wild anyways, but would be nice to fix in the future.