sd-server: set cfg_scale in the guidance parameters

[opsec-ai]

What:
sd_xl_turbo_1.0_fp16 generates low-quality images.

Why:
cfg_scale is not being passed along from sd-server to stable-diffusion
sd_xl_turbo_1.0_fp16 requires setting cfg_scale to 1.0 for good result

How:
This update passes along cfg_scale from server's json request.value to stable-diffusion's gen_params.sample_params.guidance.txt_cfg parameter.

[wbruna]

Note the /v1/images/generations endpoint follows the OpenAI spec, which does not include that parameter. You should use the parameters embedded on the prompt, which already support cfg_scale.

Also, this isn't relevant for the use with stable-ui (#1207), which uses a different endpoint /sdapi/v1/txt2img.

For sdxl-turbo, it's actually easier to pass --cfg-scale 1 to the server command line; but stable-ui doesn't have a way to request the default cfg_scale, so Guidance needs to be set to 1 on the web interface anyway.

[opsec-ai]

Superseded by #1251