feat (fastembed): Add image embedding generation

- [x] I have looked for existing issues (including closed) about this

## Feature Request

`fastembed-rs` supports image embedding generation. We should add this functionality to `rig-fastembed` because image generation can be relatively expensive on a large scale.

### Motivation

More user agency to embed their own images rather than relying on managed models.

The issue references adding image embedding generation from fastembed-rs to rig-fastembed. Start by examining the existing fastembed-rs crate for image embedding APIs and the rig-fastembed module structure. Look for integration points and existing embedding generation code. Determine what changes are needed to support image inputs, likely involving new model loading and inference logic. Testing will require image data and validation against expected embeddings.

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