[bitcoin-dev] Third version of Silent Payment implementation

[woltx]

This new version addresses most (or all) requests made in PR:

. Implements the new scheme suggested by Ruben Somsen that allows multiple silent addresses per wallet with minimal overhead.
. Implements a new RPC to retrieve silent addresses, which allows users to assign different labels to different addresses. That way, the user knows which silent address the UTXO came from.

Example:

./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress
tsp001pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kqxn48tq

# This will return the same address as above (both have no label)
./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress
tsp001pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kqxn48tq

# New label, new address
./src/bitcoin-cli -signet -rpcwallet="receiver" getspaddress 'donation'
tsp011pjgcwd9p6f2rcgf35dlgvj77h2afylg6lp5cdn0cztrk4k54w99kq80t7lt

In this new scheme, the address has a new field called identifier, which tells the receiver and sender how to set the address correctly.

If the receiver, for whatever reason, doesn't know which identifiers have been used, there is no problem. The wallet can scan all identifiers from 0 to 99. Currently, only 100 different identifiers per wallet are allowed. This limit, however, can be increased at any time in the future.

Unlike address formats so far, sp addresses are not script-related and may eventually include any additional information needed, such as an expiration timestamp (or block height). That way, users don't have to track the address indefinitely.

As usual I wrote a step by step tutorial:
https://gist.github.com/w0xlt/c81277ae8677b6c0d3dd073893210875
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