Add ability to register custom Grant Flows

[nbulaj]

Allow flexible extending of Doorkeeper with a new OAuth Grant Flows without adding them to the gem itself.

A new flow is registered like so:

Doorkeeper::GrantFlow.register :authorization_code,
  response_type_matches: 'code',
  response_type_strategy: Doorkeeper::Request::Code,
  grant_type_matches: 'authorization_code',
  grant_type_strategy: Doorkeeper::Request::AuthorizationCode

Based on #733 with additions & retro-compatibility for next extensions:

• doorkeeper_openid-connect
• doorkeeper-grant_assertions

This changes allows to fix / implement:

• Integrating SAML SSO #764 (SAML)
• Is there anyway to set the grant_type for the assertion? doorkeeper-grants_assertion#9 (Assertions)

TODO:

• Implement custom grant flows registration
• Allow multiple flows aliased with a one name in grant_flows (like in openid_connect)
• Add specs

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[joeljunstrom]

@nbulaj this is pretty neat. I had missed that this was a new thing when I fixed a bug in the openid connect lib (a bug that I think exists here in the main repo as well).

The bug consists of when a response type that requires (by spec) that the fragment response mode is used but throws an error in the controller layer will use the default (query) mode instead of the fragment for the error redirect. This is due to no checking of the response type at the controller level raise / rescue handling.

Looking at this I think it might make sense to let the grant flow strategy own the information about which response modes are available / default or MUST be used.

Would you be open to such a patch? Or are there alternative ideas on how response modes should be handled going forward?

My fix is here doorkeeper-gem/doorkeeper-openid_connect#118 but that is just fixing the already brittle monkey patching.

[rafaelsales]

@nbulaj We're about to use this feature in prod - would be nice to have this released in 5.5.0

[k-rudy]

@nbulaj thank you so much for the job.

What's the preferred place to put Doorkeeper::GrantFlow.register now? Also should we still indicate the name of the custom grant in grant_flows %w[authorization_code ...] config setting?