chore(download): Add support for segmented download

[pandomic]

Summary

Add support for multi-segmented download requests.

Breaking changes

1. In case multiple orderData entries available the attribute returns an array
2. In case data is not available orderData returns null

Linked issues

#61

TODO

• Make transactionKey() optional in case no download data available on an error occurres

[nanov]

is this still valid? can we try and test it on some platform?

[pandomic]

Hi @nanov, unfortunately I no longer have access to the real EBICS backend. If I can still remember it, it was possible to test some of the parts with https://isotestsec.postfinance.ch/corporates/?login, but it is possible it only works with Swiss phone numbers which I don't own anymore)