This change adds NewTokenSource and InitialTokenSource fields to
AuthorizationCodeHandlerConfig. NewTokenSource enables wrapping the
underlying token source to intercept token refreshes. InitialTokenSource
enables configuring the token source used by the
AuthorizationCodeHandler on initialization. Together these features
facilitate persisting OAuth2 sessions across restarts.
Includes a package example demonstrating the persistence pattern.
Fixes: #901.
See #901 for the previous discussion behind this design.
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Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
Co-authored-by: Jonathan Amsterdam <jba@users.noreply.github.com>
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