Provide a public API to instantiate enum serializer

[qwwdfsad]

In kotlinx.datetime we need a hand-rolled (because the serialization module is shipped separately) external serializer for enums (e.g. Month).

The problem is that we either have to access internal API (EnumSerializer) or re-implement both EnumSerializer and EnumSerialDescriptor from scratch

[qwwdfsad]

Note that obtaining the serializer via serializer<MyEnum>() is not an option because it does not work on JS and K/N