[Splicing] Signing utility for splicing

[optout21]

This is in preparation for Splicing (#1621 )

In splicing the channel has to sign the previous funding as an input to the new funding transactions. For this, the EcdsaChannelSigner trait has to be extended.

The proposed solution is visible in #3274, the EcdsaChannelSigner trait has been extended with this method:

	/// Create a signature for a splicing funding transaction, for the input which is the previous funding tx.
	fn sign_splicing_funding_input(
		&self, splicing_tx: &Transaction, splice_prev_funding_input_index: u16, splice_prev_funding_input_value: u64, redeem_script: &Script, secp_ctx: &Secp256k1<secp256k1::All>
	) -> Result<Signature, ()>;

with the implementation:

	fn sign_splicing_funding_input(&self, splicing_tx: &Transaction, splice_prev_funding_input_index: u16, splice_prev_funding_input_value: u64, redeem_script: &Script, secp_ctx: &Secp256k1<secp256k1::All>) -> Result<Signature, ()> {
		let sighash = &sighash::SighashCache::new(splicing_tx).segwit_signature_hash(splice_prev_funding_input_index as usize, &redeem_script, splice_prev_funding_input_value, EcdsaSighashType::All).unwrap()[..];
		let msg = hash_to_message!(sighash);
		let sig = sign(secp_ctx, &msg, &self.funding_key);
		Ok(sig)
	}

(files src/sign/mod.rs src/sign/ecdsa.rs src/ln/channel.rs)