In LangfuseConnector expose more parameters to the Langfuse client (e.g. host)

[sjrl]

It would be nice if we could expose more parameters to the underlying Langfuse client like host, threads, etc. so we can more easily customize the Langfuse client.

I think it would make sense to make some dedicated init variables like host, but for the remainder we can add a new parameter called langfuse_client_kwargs so users can pass additional options.