Fix Network -> ExtendedNetwork mapping

[Ximik]

Fixes imperviousinc/akron#29

[buffrr]

Thanks. Have you verified this works with BOTH backends yuki and bitcoin core?

[Ximik]

I've tested it in Akron. Now I've also tested it separately. It looks like it works correctly.

There is this mapping in yuki which basically makes it work

chain: match chain.network {
    Network::Bitcoin => "main",
    Network::Testnet |  Network::Testnet4  => "test",
    Network::Signet => "signet",
    Network::Regtest => "regtest",
    _ => "unknown"
}.to_string(),

But ideally we should remove these mappings at some point and make Testnet and Testnet4 separate.