Backend: implement Silent Payments sending

[webwarrior-ws]

Implemented sending of Silent Payments (BIP-352).

Created tests for Silent Payments based on test data for BIP-352.

Updated NBitcoin to version 7.0.13 because older NBitcoin
versions can't decode bech32 strings of length > 90, which is
needed for Silent Payment address decoding. Since
UnknowParameters property of BitcoinUrlBuilder is deprecated
in new version, use UnknownParameters instead. As return type
of new property is different, had to change some code.

[knocte]

With regards to the first commit message, I've noticed a pattern in the way you write them, when the commit does more than 1 thing. Let's say commit does A and B and C. You usually do this:

scope: this is some commit title

Do A because blah blah and blah (blah blah). Do B
given that blah blah blah. Do C as this is better than
blah.

Please stop using that style! If the commit contains more than one change, then do this:

scope: this is some commit title

Several things were achieved in this commit:
* A because blah blah and blah (blah blah).
* B given that blah blah blah.
* C as this is better than blah.

Or, if you prefer a prose style, then you have to link each thing A, B, C between them. For example:

scope: this is some commit title

Do A because blah blah and blah (blah blah). As a
result of this, B given that blah blah blah. Last but
not least, C as this is better than blah.

So please when listing things, either use bullets, or link each thing with conjunctions (especially if you can explain in which way A is related to B, how B is related to C or A, etc).