Feature Request: Enable use of LoRA and merger in API

[Muxelmann]

So far, I have not found a way to load LoRA/s (preferably weighted like the extension sd-webui-additional-networks) using the API.

Also, the API allows loading a new checkpoint using sd_model_checkpoint with {url}/sdapi/v1/options, but if one wants to merge two models and load it, the merging must be done using WebGUI.

It would be great to allow setting of LoRA/s with the options endpoint, and have an additional endpoint like {url}/sdapi/v1/merger for merging two checkpoints. The data for the latter may be:

merger_payload = {
    "model_a": "<model-name> [<model-sha256>]",
    "model_b": "<model-name> [<model-sha256>]",
    "multiplier": 0.5,
    "output_name": "<model-name>",
    "format": ".ckpt",
    "interpolation": "weighted-sum",
    "config": "A, B or C",
    "float16": False
}

[ekeric13]

Seconding adding checkpoint merger to the api