Refactor the SDK to support the extension grant natively

[thivi]

There can be a scenario where the ID token obtained from an identity server needs to be exchanged with another server to get an access token from it. This access token will be used for accessing protected resources.

This is possible with the requestCustomGrant method. However, this does not persist the session information from both the servers.

Since applications might require session information from both the servers, there should be a way to persist both the session information.