Add keep alive field into ollama requests.

[s-kostyaev]

We need to add "keep_alive" option to ollama requests to be able to control model lifecycle in memory.

https://github.com/ollama/ollama/blob/main/docs/faq.md#how-do-i-keep-a-model-loaded-in-memory-or-make-it-unload-immediately

Relates to s-kostyaev/ellama#160

[s-kostyaev]

We can't use :default-chat-non-standard-params here because this option at the top level of request object.

[ahyatt]

I think we could probably use it if we specially handle this particular variable in making the request. I'll take a look at this soon.