Reimplement GetVariables based on new segments layer

[Philipp91]

This PR adds flexible parsing for HITANS segments version 1 through 6 and exposes the parsed segments with a common interface. This allows reimplementing GetVariables:parseTanModes() (which is a business logic layer function) without any interspersed parsing logic. If necessary, the common interface allows adding other HITANS versions without much thinking or danger of breaking other ones. The unit test remains unchanged besides plumbing.

@roben I'd appreciate your review, as you are probably most familiar with how HITANS works across the various banks. I'm not sure if your unit tests cover all the special cases that you considered when implementing the code. In particular, I see a comment about Sparkasse declaring version 6 and you suspecting that they send version 5, yet the unit tests still works with my HITANSv6 class -- I haven't even implemented HITANSv5.

[nemiah]

The changes look sane to me and I'd give them a go.

@roben What do you say?

[Philipp91]

Found this in the specification:

B.5.1 Geschäftsvorfall HKTAN in Segmentversion #6
In der BPD können sich mehrere Segmentversionen von HITANSSegmenten befinden, wobei den einzelnen HITANS-Segmenten
über das Element „Sicherheitsfunktion, kodiert“ unterschiedliche
Verfahren zugeordnet sein können. Ein Kundenprodukt sollte – beginnend mit der höchsten Segmentversion – alle in der BPD enthaltenen HITANS-Segmente analysieren, um so dem Kunden alle vom
Kreditinstitut unterstützten Sicherheitsverfahren anbieten zu können.
Beispiel: Die BPD enthält Definitionen für HITANS#6 und
HITANS#5. In HITANS#6 gilt für starke Kundenauthentifizierung
analog PSD2, mit HKTAN#5 ist übergangsweise auch noch eine
Dialoginitialisierung ohne starke Authentifizierung möglich.

tl;dr: Need to read all HITANS versions, though any version less than 6 is no good for PSD2 authentication. That said, not everything needs that strong authentication nowadays, e.g. stock/broker functions do not. So I have now implemented the other HITANS versions as well and removed findLatestSegment() after all (it's still in commit f7d6bb0161a29ff6c3e231aa3a60c2f858bd3638 if anyone needs it).

[roben]

Looks solid to me, too. Thanks for all the work @Philipp91!