InteractiveIDTokenSource returns an `IDTokenSource` which performs an interactive Oauth token flow in order to retrieve an `IDToken`.

NewPKCE creates a new PKCE challenge for the specified provider per its supported methods (obtained through OIDC discovery endpoint).

StaticIDTokenSource returns an `IDTokenSource` which always returns the given `IDToken`.

PKCES256 is the SHA256 option required by the PKCE RFC.

IDToken is a structured representation of an OIDC IDToken.

PKCE specifies the challenge and value pair required to fulfill RFC7636.

IDTokenSource provides `IDTokens`.