add a getter for id_token for OpenID Connect

[jsifuentes]

Type: new feature

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• I have read and understood the Contributing Guidelines.
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Small description of change: Adds a getter for id_token when using OpenID connect.

When using 'RP-Initiated Logout' in OpenID, the RP can redirect the user to the Identity Provider and provide a id_token_hint which is meant to equal a previously issued id_token. Spec. But when using this library, the id_token is parsed and the original id_token string is discarded here. Only the parsed JWT is left over, and it doesn't seem possible to reconstruct the id_token string from the JWT class. So this PR saves the id_token string in AccessToken and provides a getter for it.