I could see `client_id` is defined in the spec,
however it seems this specific use is only for Instagram,
so I would rather put this feature into Instagram directly.

However, indeed there might have some other OAuth2
providers need this. Then we could introduce ClientOauth2
like ClientOauth1: https://github.com/godfat/rest-core/blob/master/lib/rest-core/client_oauth1.rb
to provide this kind of stuffs.