feat:serializers cast custom types with msgspec

[Str1kez]

When working with FastStream and the msgspec serializer, I encountered an issue where I couldn't retrieve custom types specified in the consumer's Struct from JSON.
Unfortunately, when manually publishing, it's impossible to specify how to make custom types serializable, since the encode method is static and doesn't take the MsgSpecSerializer state. However, there is a workaround: manually executing msgspec.json.encode with enc_hook when calling await broker.publish.

As far as I know, there's no easier way to cast types in msgspec.
https://jcristharif.com/msgspec/extending.html