feat: add example Python web client

[wbruna]

Adds a basic Python command-line client for the sd-server API.

I've tried to keep it compatible with the sd-cli command line, to make it easier to use from user scripts already expecting that interface; so unsupported arguments are ignored, and the server URL can be specified with an environment variable. It uses supported API fields when available, and propagates all other server supported flags via sd_cpp_extra_args.

It's mostly useful for testing the server itself, and as reference code for third-party implementations.

[leejet]

In fact, since the API is OpenAI-compatible, you can directly use the OpenAI Python SDK to call it. Below is a simple example.

from openai import OpenAI
import base64

client = OpenAI(
    api_key="local-api-key",
    base_url="http://127.0.0.1:1234/v1"
)

result = client.images.generate(
    model="sd.cpp",
    prompt="a lovey cat",
    size="512x512"
)

image_base64 = result.data[0].b64_json
image_bytes = base64.b64decode(image_base64)

with open("output.png", "wb") as f:
    f.write(image_bytes)

print("done")

[wbruna]

I've intentionally avoided extra dependencies, to make the script available for any base Python install.

I could add a test for the OpenAI module, and keep the current code as a fallback.