Add REST API auth flow for the new UI

[ephraimbuddy]

This PR is to provide authentication to the new UI. The implementation uses the existing Flask Appbuilder authentication method configurations.
First, the react app checks the configuration on the server by sending a GET request to api/v1/auth-info. This endpoint returns information about the configured auth method. With the information, the react app can then present a login form to the user if necessary.

Below I explain the flow for OAuth

OAuth
This authentication uses two endpoints, one is used to get the authorization URL while the other is used for verification and sign-in. The react app makes a GET request to api/v1/auth-oauth/{provider} with the provider name and redirect url e.g api/v1/auth-oauth/google?redirect_url=http:localhost:5000, the endpoint responds with authorization URL which the react app will redirect users to.
When the user is authenticated by the provider, the provider will redirect user to the redirect_url provided by the react app. The react app should handle the redirection, and make a GET request to server at api/v1/oauth-authorized/{provider} with the state returned from the provider. If everything is OK, user is logged in and user data is returned.

The one advantage we can get using the refresh token is to get ride of auth_backend.

OpenID is not currently implemented.

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This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[msumit]

I think type_mapping can be a constant & be out of this function.

[msumit]

Don't we need more status codes like 50X which happens when the auth configs are wrongly setup & callback fails?

[msumit]

Couldn't understand the logic here or maybe I misinterpreted it. How is a user going to provide the token of another user?

[ephraimbuddy]

Thanks for the reviews, @msumit, I will revisit this and may have to open a new PR and address a lot of things

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This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.