Improve public API: `_serializer_any` behave like I'd expect `any_to_pb` to behave, perhaps the names should be swapped

[chenkasirer]

_serializer_any and _deserializer_any actually deal with ANY (including lists and dicts, as well as known types, whereas any_to_pb and any_from_pb only handle known types.

[WeiTing1991]

This was addressed in PR #83 (commit 6939999), which added explicit list_value/dict_value oneof arms to AnyData.

The two-layer dispatch is now intentional:

• _serializer_any / _deserialize_any — internal dispatchers that handle the full type universe (lists, dicts, known types, primitives)

• any_to_pb — public API that resolves the serializer for a single object, with fallback and primitive_to_pb

• any_from_pb — public API that does the reverse

The naming is slightly misleading since they don't handle "any" (no lists/dicts), but the behavior and layering are correct.
Maybe just the naming issue. we could change it to object_to_pb or item_to_pb ... but it is going to have breaking change if other repos use it.
@chenkasirer Maybe we can keep like this. since the any seems a bit loose.