Allow adding fields from user info

[MartinWallgren]

This change makes it possible to extract extra fields form user info to the generated token.

For example, to have upn from the Azure token be added to the final JWT, we can add the below to the oauth config section.

extract upn from userinfo

This PR is an attempt to solve this https://github.com/greenpau/caddy-security/issues/190

[MartinWallgren]

@greenpau any feedback on this? This is a real problem for most that are using AzureAD. I don't mind implementing something else if you don't agree with this solution.

[greenpau]

This is a real problem for most that are using AzureAD.

@MartinWallgren , I do not have time to look at this at the moment. The idea sounds good. Just need to find some time to do maintenance of the plugin. Please ping me again if it is not merged by the end of august.

[greenpau]

thank you for the contribution. What is the effect of this? Is it possible to only add "select" fields as opposed to everything?

@MartinWallgren , please see ^

[raffaeleguidi]

hi, any plan on adding this to a release?