add keep_alive option

[0xRavenBlack]

• add keep_alive option (controls how long the model will stay loaded into memory following the request (default: 5m)) Ollama API Description

[stavsap]

@0xRavenBlack can you please add description to https://github.com/stavsap/comfyui-ollama/blob/main/CHANGE_LOG.md?

[0xRavenBlack]

@stavsap ok, its done :)