Add parameters for refiner in txt2img API

[danielemaddaluno]

I propose to add these parameters to the txt2img (these parameters are available in the current sd API).

These should be the missing parameters to have the whole possible payload (check the doc):

refiner_checkpoint=None,
refiner_switch_at=0,
disable_extra_networks=false,
comments={},
hr_checkpoint_name=None,
hr_sampler_name=None,
hr_prompt="",
hr_negative_prompt=""

[rstaubh]

Is there any update for setting the refiner via webuiapi?