Add Struct.__struct_config__ attribute - #328

This adds a new `__struct_config__` attribute to struct classes and
instances. This attribute holds a (dynamically created)
`msgspec.structs.StructConfig` instance describing the configuration
settings for the respective struct type. Note that these settings are
immutable; this object is only useful for type introspection, not for
modifying settings of existing types.

Previously these settings were exposed through private (and
undocumented) attributes. Those attributes have all been removed in
favor of this new public interface.

```python
class Point(msgspec.Struct, frozen=True):
    x: int
    y: int

assert Point.__struct_config__.frozen
```