Add support for ECDSA signing

[AshUK]

See https://www.rfc-editor.org/rfc/rfc8392#appendix-A.2.3 of the spec.

I'd imagine API akin to:

const cwt = new Cborwebtoken()
const signed = cwt.sign(payload, privateKey).then((token) => {
	console.log(`cwt length: ${token.length}`)
	console.log(`cwt value : ${token}`)
})