export_strategy: Add strip_default_attrs to ExportStrategy and it's export_fn signature.

This allows for a single place (ExportStrategy) where the config for strip_default_attrs is stored. If the export_fn callback accepts the `strip_default_attrs` argument, it can
then propagate the `strip_default_attrs` arg to estimator.export_savedmodel().

Also changed the default for `strip_default_attrs` in the ExportStrategy factory methods (saved_model_export_utils.py) to None. This allows clients that process a given ExportStrategy object to determine if a value has been provided for ExportStrategy.strip_default_attrs field (i.e., `is not None`).
If no value has been provided, those clients can then use whatever default they wish to use.

PiperOrigin-RevId: 182217376