Add option to add __dict__ on msgspec.Struct types #324

Description

Currently msgspec.Struct types are __slots__ only, defining no instance dicts. This is for both efficiency (slots reduce memory usage and require no dict-lookup for internal access) and correctness (no instance __dict__ means you get a runtime error for typos when setting attributes). However sometimes it can be useful to have a lazily-created __dict__ defined. One use case is functools.cached_property support, which only works for objects with a __dict__ defined (#204).

We should enable this as an opt-in feature. Something like:

class Point(Struct, dict=True):  # probably a better option name
    x: int
    y: int

# both `x` and `y` are still stored in `__slots__`, no `__dict__` allocated yet
p = Point(1, 2)

# Setting an undeclared attribute will allocate a `__dict__` and store the attribute there
p.z = 3

# undeclared attributes are ignored by msgspec's encoders
msgspec.json.encode(p)
# {"x":1,"y":2}