First idea of a extendable JWT decoder

[anakinj]

An idea of how custom decoding/encoding of an JWT token could be handled. Related to #428

Was thinking that custom decode modules could be created by including the JWT module into them. This included module could then provide a set of DSL methods where different handlers could be defined. For example:

module MyCustomJWT
  include JWT

  algorithm 'HS256'

  decode_payload { |header, payload, signature| payload.reverse }
  encode_payload { |payload| payload.reverse }
end

payload = MyCustomJWT.encode({'pay' => 'load'}, key: 'secret')

decoded_data = MyCustomJWT.decode(payload, key: 'secret')

This approach would also make defining keyfinders and such much more elegant than passing everything to the ::JWT.decode method

In this PR only a super simple decode_payload logic has been implemented as an illustration how it could work.

[sourcelevel-bot]

JWT::Extension::ClassMethods has no descriptive comment

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[sourcelevel-bot]

JWT::Extension::ClassMethods#decode refers to 'segments' more than self (maybe move it to another class?)

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[sourcelevel-bot]

JWT::Decode#decode_and_parse_payload calls '@options[:decode_payload_proc]' 2 times

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[sourcelevel-bot]

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