client: Allow Firo to send to an EXX address.

[dev-warrior777]

- EXX addresses are used for Firo and other privacy coins to
  send funds to  Binance and other exchanges due to last US
  administration regulations.

• This PR makes a non-standard P2PKH script to enable this.

[dev-warrior777]

Dex Electrum Firo => Binance

[dev-warrior777]

Working using Dex Electrum Firo wallet. And can send to Binance and to Firo full node.

• Todo: Test on Dex Firo full-node.

[dev-warrior777]

Dex full-node => Binance Tx

2024-12-12 14:53:07.987 [DBG] CORE[firo]: 2 signature cycles to converge on fees for tx 48447697517d82f177143962b8c98904208bd09c8ff743bd4672198c5ed1a824: min rate = 1, actual fee rate = 1 (226 for 226 bytes), change = true
2024-12-12 14:53:08.000 [INF] CORE: notify: |SUCCESS| (send) Send successful - Sending 0.33300000 firo to EXXKcAcVWXeG7S9aiXXGuGNZkWdB9XuSbJ1z has completed successfully. Tx ID = 48447697517d82f177143962b8c98904208bd09c8ff743bd4672198c5ed1a824:0

[buck54321]

I was looking at an older commit of yours when I started this, but check out this variation please.

buck54321/dcrdex@0030e49...buck54321:dcrdex:exx-addr-interface

[dev-warrior777]

I was looking at an older commit of yours when I started this, but check out this variation please.

buck54321/dcrdex@0030e49...buck54321:dcrdex:exx-addr-interface

Thanks for this. The logic did not change between c6db64d and 7a86357, just additional tests & cleanup, so your suggestion is welcome.

Please believe me when I say I considered making an interface to identify the user of the PayToAddrScript(addr) before I went the other route. My concern was always to minimize any footprint in common code and your code does this better. It is also less C-like than mine and solves the issue of the address encoding information being "lost" in btcutil.Address . More elegant.

Let me take a look today and send some more Firo to Binance. I also want to keep a couple of other currently unused byte constants for normal transparent addresses around for future use.