Remove LLAMA_CLOUD and warn users to switch to TOGETHER or another provider

[lalalune]

We have deprecated LLAMA_CLOUD since we have a lot of llama cloud options now-- it's called TOGETHER instead

Add a character file validation check to warn the user that "llama_cloud" is no longer an option and they should switch to together -- let's warn but do the change so it doesn't error, like a deprecation warning.