German Middleware behaves differently when switching to late-signing mode

[TSchmiedlechner]

Describe the bug

When using the late signing mode (via the ftReceiptCaseFlag 0x0000000000010000), the Middleware behaves slightly different in Germany than it does in France or Austria.

• In Austria and France, the Middleware always returns the ftState 0x...8 when it's operating in late signing mode - no matter if the following receipts have the ftReceiptCaseFlag 0x0000000000010000 (before the zero receipt is sent)
• In Germany, this ftState is only returned when the mentioned flag is included into the receipts

We should IMO change the German behavior to match the Austrian and French one, because the Middleware in fact obviously stays in late signing mode.

To Reproduce

1. Send a POS receipt to the Middleware where the ftReceiptCase has the flag 0x0000000000010000
2. The response will have the expected ftState 0x...8
3. Send another POS receipt without the flag 0x0000000000010000
   1. In Austria and France, the response will have the ftState 0x...8
   2. In Germany, it will have the state 0x...00

I'm a bit worried though because - depending on the implementation - this could be a breaking change, although IMO that's very unlikely. It may also make sense to fix this with the tagging v2 🤔

[volllly]

• [x] Update docs to correctly reflect how the late signing mode works
• [x] Add to the docs how behaviour in germany currently differs