feat(jose-jwk): JWK Thumbprint

[jakelee8]

Implement RFC 7638 JWK thumbprint support in the jose-jwk crate.

Closes#89

[tarcieri]

Is there some reason this can't just go in jose-jwk instead of being its own crate? It could potentially even be feature-gated

[jakelee8]

Is there some reason this can't just go in jose-jwk instead of being its own crate? It could potentially even be feature-gated

@valkum suggested a separate crate in #89 (comment)

Implementing JWK Thumbprint in jose-jwk works better with the non-exhaustive Key enum.