Ability to extend JWT encoding and decoding behaviour

[anakinj]

This is a second attempt of making it possible to extend the token decoding/encoding behaviour. First version of the same idea in #434

For example to support deflating the payload after verification as discussed in #428

Rough example on how it works

class MyJWTHandler
  include ::JWT

  algorithm 'ES256'

  jwk_resolver do |_options|
    # Load JWKs from trusted source
  end

  signing_key es256_private_key

  encode_payload do |payload|
    ::Base64.urlsafe_encode64(::JWT::JSON.generate(payload)), padding: true)
  end

  decode_payload do |raw_payload|
    ::JWT::JSON.parse(::Base64.urlsafe_decode64(raw_payload))
  end
end

Focus now was on the decoding/encoding and signing of the token. The mechanics could be extended to claims etc..

[anakinj]

Im going to refine this a little bit more. Not super happy with the methods setting the signing keys with.

[anakinj]

Im going to close this one and yet again try to tackle issues one by one.

First up is this #510