Add Custom API Endpoint for Ollama

[hhyqhh]

I have added a custom endpoint for Ollama:

Endpoint = "http://xxxx/api/chat";
chat = ollamaChat("qwen2:72b",systemPrompt,Endpoint = Endpoint);
txt = generate(chat,"what is your name")

[ccreutzi]

Thank you for the proposed change. Since we have the same effect for the end user in #53 already, I hope you don't mind if I close it anyway. (The other PR also contains tests and doc changes.)