feat: Allows Key instance to be used for encoding

[Oinkling]

Allows an instance of Key to be passed as the second argument to JWT::encode and JWT::sign, making $alg optional.

This is more in line with how decoding works and as such it makes sense to streamline it.

This also generally makes the process easier in the use case where you get an instance of Key injected so you can just encode using JWT::encode($payload, $key) rather than JWT::encode($payload, $key->getKeyMaterial(), $key->getAlgorithm())

[bshaffer]

this looks good, and I can see the logic in it... but I am not thrilled about having two ways of doing the same thing. It would be less confusing, looking at the phpdoc, to just have the one way (as it is today).

But I don't have a real issue with it. Could you add some unit tests as well?

[bshaffer]

nit: this library prefers the use of instanceof

Suggested change

	if (is_a($key, Key::class)) {
	if ($key, instanceof Key::class) {

[bshaffer]

We would also want to throw an exception if $alg isn't null. Otherwise this would be possible and potentially cause confusion:

JWT::encode($payload, $key, 'FooAlg');