Add ControlNet example truss

[spal1]

This PR adds a ControlNet truss to the examples/ directory. This truss in particular is created for the sd-controlnet-scribble model.

Input images are expected to be PNGs serialized as base64 and output images are also PNGs serialized as base64.

Trusses can easily be made for the other StableDiffusionControlNetPipeline models listed here

You can test this truss by running the following:

import truss
from shared.base64_utils import b64_to_pil

controlnet_truss = truss.load("controlnet-scribble")

# The example uses this input image: https://huggingface.co/lllyasviel/sd-controlnet-scribble/resolve/main/images/bag.png
bag_input = controlnet_truss.example("bag example").input
response = controlnet_truss.docker_predict(bag_input)

# The outputted image will be saved as "bag_output.png"
b64_to_pil(response["images"][0]).save("bag_output.png")

[bolasim]

Can you add this PR description to a README in that folder so people know how to use it?

[joostinyi]

tested readme examples – working well