Multi system message constructor, and types for model + chat roles.

[WillMatthews]

While using this library, I found myself using NewXXXTextMessage, NewUserImageMessage a lot, but wondered why there was no similar constructor for multi-system messages. This PR adds this functionality with a test.

This PR also adds a type for model names and chat roles, since both of them were plain strings. It's just a string, so it doesn't offer any "protection", but is nice for sanity checking while writing code / reasoning about correctness.