native_proto_caster.h: Add `*_with_smart_holder_support_enabled` specializations.

[copybara-service[bot]]

native_proto_caster.h: Add *_with_smart_holder_support_enabled specializations.

The

• copyable_holder_caster_shared_ptr_with_smart_holder_support_enabled and

• move_only_holder_caster_unique_ptr_with_smart_holder_support_enabled

SFINAE helpers were introduced with https://github.com/pybind/pybind11/pull/5257. They need to be specialized here (std::false_type) because native_proto_caster.h has its own specializations for

• copyable_holder_caster<ProtoType, std::shared_ptr<ProtoType>> and

• move_only_holder_caster<ProtoType, std::unique_ptr<ProtoType>>.