Support for local/in-process embedding libraries

[titusz]

For offline or cost-free embeddings, local libraries like FastEmbed
or sentence-transformers are useful. These run in-process without HTTP.

Possible API

client = Client()
response = client.embed(
    model="fastembed/BAAI/bge-small-en-v1.5",
    inputs=["Hello world"]
)

This would need a different provider type (no HTTP client) and could be an optional
dependency to keep the core package light.

Happy to discuss if this fits the project scope. It's a different paradigm from
API-based providers but would be quite handy for local development and testing.

Thanks for building catsu!