Integration of OAuth2 flows in Feign

Hello @velo

I am starting working on the integration of OAuth2 client flows into Feign. I am opening this issue first.

Reason one: to evaluate the public interest for this feature.

Reason two: as this is not an easy task, I will need to ask for your opinions and suggestions during the development.

Are you agreeing to proceed like that? Or do you prefer to move discussion on Gitter?