feat: add MuAPI image generation service

[Anil-matcha]

Summary

Adds `src/pipecat/services/muapi/` — an image generation service backed by muapi.ai, a unified API aggregator for 400+ generative media models including Flux, Midjourney, GPT-4o Image, Google Imagen, Seedream, HiDream, Reve, and more.

What this adds

• `src/pipecat/services/muapi/init.py`
• `src/pipecat/services/muapi/image.py` — `MuApiImageGenService` implementing `ImageGenService`

The service follows the same architecture as `FalImageGenService`:

• Accepts `aiohttp_session` + `api_key` + `model` + `settings`
• Implements `run_image_gen(prompt)` → `AsyncGenerator[Frame, None]`
• Handles muapi's async submit → poll pattern internally

Usage

```python
import aiohttp
from pipecat.services.muapi.image import MuApiImageGenService

async with aiohttp.ClientSession() as session:
image_service = MuApiImageGenService(
aiohttp_session=session,
api_key="your-muapi-key",
model="flux-schnell", # or midjourney, gpt4o, imagen4, seedream, etc.
)
```

Supported models

`flux-schnell`, `flux-dev`, `flux-kontext-dev/pro/max`, `hidream-fast/dev/full`, `midjourney`, `gpt4o`, `gpt-image-2`, `imagen4`, `imagen4-fast`, `seedream`, `reve`, `ideogram`, `hunyuan`, `wan2.1`, `qwen`

Get an API key at muapi.ai/dashboard/api-keys.