Login the user manually and return the token on headers by using a POST endpoint (facebook provider)

When posting issues, please include the following information to speed up the troubleshooting process:

* **Version**: 1.1.4
* **Routes**: are you using some crazy namespace, scope, or constraint? no
* **Gems**: ActiveAdmin, but does not have to do with the issue
* **Custom Overrides**: what have you done in terms of [custom controller overrides](https://github.com/lynndylanhurley/devise_token_auth/#custom-controller-overrides)? None
* **Custom Frontend**: are you using [ng-token-auth](https://github.com/lynndylanhurley/ng-token-auth), [jToker](https://github.com/lynndylanhurley/j-toker), [Angular2-Token](https://github.com/neroniaky/angular2-token), or something else? None

Hello, this is an open question that I had to write a custom controller to log in to my user using the access token. Imagine the scenario:

1. You have a mobile app (react-native) to log in with Facebook
2. You use the mobile tools to do the OAuth login and get the access-token and don't need to use the omniauth regular routes to get the token
3. You need a way to log in the user on the Rails API using omniauth-facebook, but you already have the access-token.

This scenario is becoming very common since you don't need to use the whole omniauth flow on mobile apps.

Basically, it would be very nice to have a `POST` route that you can just pass the `access-token`, and the application would log in the user.

I had to create a custom facebook controller and use some of the internals of https://github.com/lynndylanhurley/devise_token_auth.

I am not sure if this question is more related to this repo or not. Anyway, here's you can find a way to log in just by using the access token by adding a field to the user model:

hacky by fancy way:

```ruby
class FacebookController < ApplicationController
skip_before_action :verify_authenticity_token

def create
facebook_data = HTTParty.get("https://graph.facebook.com/me", query: {
access_token: params[:access_token],
fields: 'email,name,id'
}).parsed_response

@user = User.find_by(facebook_id: facebook_data["id"])

if @user
@auth_headers = @user.create_new_auth_token

response.set_header('access-token', @auth_headers['access-token'])
response.set_header('token-type', @auth_headers['token-type'])
response.set_header('client', @auth_headers['client'])
response.set_header('expiry', @auth_headers['expiry'])

render json: @user
else
render json: { error: "user not found", status: :not_found }
end
end
end
```

Open question: Is it possible to POST to any route nowadays using a specific param with the access token and the gem would already login the use and return the headers, etc?

Start by reviewing the custom FacebookController example and the devise_token_auth internals it uses, along with the existing OAuth routes. Define whether a POST endpoint accepting a Facebook access token belongs in this repository and what authentication headers and user states it must return; no repository file or test is named in the issue.

Written by the indexing model from the issue text.