Add msgspec support

[dhirschfeld]

Presentation of the new feature

Allow using msgspec as an alternative to pydantic

• https://jcristharif.com/msgspec/jsonschema.html

Where does it fit in Outlines?

Similarly to pydantic support.

Are you willing to open a PR?

I won't have the time to add this myself as it's more of a nice to have than a requirement for me personally.