Allow the OpenAI API BASE_URL to be overridden

[chris-hatton]

Projects such as LocalAI emulate the OpenAI API's.

To enable the use of YChat with such services, it's necessary to allow for the host (and maybe port) to be overridden when creating a YChat instance.

For example:

val ychat = YChat.create(apiKey = "<api key>", host = "127.0.0.1", port = 8080)