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`:
```python
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)
```