Support custom builtin_types in to_builtins

[jcrist]

This adds support for "custom" builtin_types in to_builtins. This is useful when wrapping a protocol like bson that has protocol specific types like ObjectId that you'll want to pass through unchanged.

For example, to support passing bson.ObjectId through msgspec.to_builtins:

In [1]: import msgspec, bson

In [2]: class Point(msgspec.Struct):
   ...:     _id: bson.ObjectId
   ...:     x: int
   ...:     y: int
   ...:

In [3]: obj = Point(bson.ObjectId(), 1, 2)

In [4]: obj
Out[4]: Point(_id=ObjectId('64dad4a21ffe3dbdae50a48e'), x=1, y=2)

In [5]: msg = msgspec.to_builtins(obj, builtin_types=(bson.ObjectId,))

In [6]: msg
Out[6]: {'_id': ObjectId('64dad4a21ffe3dbdae50a48e'), 'x': 1, 'y': 2}

In [7]: msgspec.convert(msg, type=Point)
Out[7]: Point(_id=ObjectId('64dad4a21ffe3dbdae50a48e'), x=1, y=2)

Fixes #516.