feat: add plugin interface for custom embedding providers

[RobertLD]

Only three hardcoded providers are supported. Users should be able to register custom embedding providers.

Suggested implementation:

interface EmbeddingProviderPlugin {
  name: string;
  embed(text: string): Promise<number[]>;
  embedBatch(texts: string[]): Promise<number[][]>;
  dimensions: number;
}

registerProvider(name: string, factory: () => EmbeddingProviderPlugin): void;

This would allow Cohere, Anthropic, Azure OpenAI, Hugging Face, and other services.