[OAuth2] Add support for passing `state` to Assent

[mike1o1]

Currently, we can use the authorization_params to pass parameters to Assent, such as scope or access_type. Previously, we could also pass in state, and Assent would either use it directly or generate it for us if passed in.

In Assent 3.0 however, passing in State has been deprecated.

It would be great if we could perhaps have another option in the OAuth2 configuration to provide state.

In addition, it would be nice if we could find a way to get that state and somehow pass it to the actions opts - maybe in the context?

[mike1o1]

I am happy to try and work on this, but I think I'll need some guidance on the second part. I think passing state directly should be pretty straight forward.

[jimsynz]

Okay, looking at the changes in assent we need to modify the OAuth2 entity/struct to contain a state option which defaults to false and then modify OAuth2.Plug to include it in the generated config (see config_for/2).