Store original encoded and signed identity JWT in OAuth2Credentials

[clancychilds]

For some authorization flows, including the Auth0 Delegation flow, the endpoint requires the original signed identity JWT to be sent. To support this, it is necessary to store the JWT alongside the decoded content.

This PR adds id_token_jwt as an element in the OAuth2Credentials. It also adds the logic to populate that element in parts of the flows that receive the identity token.

Resolves: #678

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[clancychilds]

Argh. Closing this because I had the wrong credentials on the commits.