Reuse the Network type existing in upstream

[yanganto]

It is great not to duplicate definitions of the same resource.

ldk-node/src/types.rs

Lines 144 to 153 in c755eee

	pub enum Network {
	/// Mainnet Bitcoin.
	Bitcoin,
	/// Bitcoin's testnet network.
	Testnet,
	/// Bitcoin's signet network.
	Signet,
	/// Bitcoin's regtest network.
	Regtest,
	}

This crate depends on bdk and os as bitcoin.
The Network is already defined in bitcoin crate.
https://docs.rs/bitcoin/0.30.1/bitcoin/network/constants/enum.Network.html

[tnull]

We couldn't for some time due to rust-bitcoin making it non_exhaustive and our bindings generator not supporting that. We will however drop our newtype again in #230.