Add SAML 2.0 Grant Type support

[jzheaux]

As detailed in RFC 7522. It would be nice for institutions needing to formulate a bridge between SAML 2.0 authentication and OAuth 2.0 authorization.

[jgrandja]

Related gh-1270

[opcooc]

Hi, are you considering starting to implement this issue.

[jgrandja]

@opcooc Not at the moment since there is very little demand for this feature given there is only 1 upvote.

[opcooc]

@jgrandja OK, Additionally, do you have a general implementation idea? If so, could you share it? I would like to try it out locally.Thank you.

[jgrandja]

@opcooc No I don't have a general implementation idea. Honestly, I haven't even read the spec.

[opcooc]

@jgrandja OK, Thank you.

[riccardolunghi]

Hello, I tried by myself and I've been able to easily configure the login to an external saml identity provider.
The problem is that the application expects in return from the login flow an authorization code that the saml login doesn't supply.
Any suggestion on how to add the authorization code generation after the saml login?
Adding this part the client could continue the authorization flow and exchange the authorization code for an access token as usual.