Automatic JWK enrichment

[bellebaum]

This PR will allow to derive most standardized common parameters from already specified parameters.
This is a follow-up to a few points which could not be addressed in #520, but were brought up by me in #518.

Overview

Using all of its features, given an X.509 certificate, you can acquire a JWK like this:

::JWT::JWK.new(certificate)

This will extract the public key, construct the JWK, and try to set the following parameters:

• kid as previously implemented
• x5c is a representation of the certificate
• x5t and x5t#S256 are wide-spread certificate thumbprints
• key_ops and use state allowed key usages and are derived from the X.509 keyUsage extension, if present
• alg will be set to a sensible default depending on use and kty

Of course, any parameters specified in the above invocation will still take precedence.

There is even support for the x5u parameter, to fetch a remote certificate chain:

x5u_fetcher = ->(uri) {
    # Fetch uri, return as Array of OpenSSL::X509::Certificate
}
::JWT::JWK.new(jwks_hash, nil, enrich_key: true, x5u_handler: x5u_fetcher)

TODO

• Proper certificate validation
• Check if code from x5c_key_finder can be reused
• new README examples
• Update changelog

[anakinj]

I realize this is a WIP but I got very curious so will drop a few comments.

[anakinj]

Im wondering about where this certificate specific stuff should live. Now the logic baked into each and every JWK object created.

Most of these methods are just setting some values under certain keys? Could it just be a separate mechanism (method/extension) that would build the params passed into the key or add the params to an existing key?