Make HTLCDescriptor writeable

[davidcaseria]

Because HTLCDescriptor is used in the EcdsaChannelSigner trait it is may be needed by an external signer, thus it should be able to be serialized. Implementing the existing Readable/Writeable traits seems to make the most sense.

[TheBlueMatt]

We really want to deprecate the writing of signers - ideally modern signing implementations write out zero bytes and re-derive their state entirely from the derivation info.

[davidcaseria]

Sorry @TheBlueMatt I should've been more clear. HTLCDescriptor does not need to be serialized for the state of the external signer, it needs to be serialized since it is used in a function call by the EcdsaChannelSigner trait:

rust-lightning/lightning/src/sign/mod.rs

Lines 502 to 504 in f6a4505

	fn sign_holder_htlc_transaction(&self, htlc_tx: &Transaction, input: usize,
	htlc_descriptor: &HTLCDescriptor, secp_ctx: &Secp256k1<secp256k1::All>
	) -> Result<Signature, ()>;

The data passed to the function needs to be serialized so that it can be sent to the external signer.