Bitcoin Signet support

[chill117]

Add new network: "Bitcoin (signet)". For more details:
https://en.bitcoin.it/wiki/Signet

Signet does not appear to be widely supported yet.

Faucets:

• https://signetfaucet.com/

Block explorers:

• https://explorer.bc-2.jp/
• https://signet.btc21.org/

Web services:

• esplora:
  • https://explorer.bc-2.jp/

Broadcast transaction services:

• ?