Support transient identities and traits

[khvn26]

Update the get_identity_flags interface:

    def get_identity_flags(
        self,
        identifier: str,
        traits: typing.Optional[typing.Mapping[str, TraitValue]] = None,
+       *,
+       transient: bool = False,
+       transient_traits: list[str] | None = None,
    ) -> Flags:

Update the generate_identities_data utility to add values according to updated schema here Flagsmith/flagsmith#4279