Allow fixed-length decimal amount representation

[bigspider]

Currently, we use the truncated amount representation for transaction amounts, e.g.:

0.0012 BTC

In order to facilitate thinking about amounts in smaller units (sats) some wallets started representing the decimals in a fixed 8-digits length, with spaces in between:

0.00 120 000 BTC

This has the advantage to allow reading the amount in sats as a normal number, with the usual grouping of 3-digits at a time, for amounts below 1 BTC.

Allowing the software wallet to customize the visualization to match the one they use would improve the UX and avoid confusion.

[bigspider]

Since version 2.1.2, #133 is merged, which allows using a non-zero p1 byte (currently ignored). Therefore, we could reserve (for example) the two lowest-significant digits of p1 for the preferred format

• 00: use whatever is Ledger's default
• 01: truncate (e.g.: 1.23456 BTC)
• 10: fixed-length (e.g.: 1.23 456 000 BTC
• 11: ERROR (undefined, shouldn't use!)

(perhaps we should reserve 3 bits to leave room for future formats)

Then in client libraries, we could use this logic:

• if version <2.1.2: set p1 := 0 (other values would error!)
• otherwise: pass the flag as requested from the software wallet (it will be gracefully ignored in versions before this feature is merged).

That could be an optional parameter in the sign_psbt command.