Change ModelProvider so we don't have to manually add support

[JonahSussman]

Right now, we have to add each provider manually, ex #557

We should probably think about doing something along the lines of dynamically looking up the class provided inside the langchain module instead of manually adding all these classes. Something like:

ModelClass = getattr(langchain, the_class_name)

One wrinkle is that some models come from different modules:

from langchain_aws import ChatBedrock
from langchain_community.chat_models import ChatOllama

So it needs to be slightly more clever than that.