Embeddings in v1: init_embeddings and provider packages

# Embeddings

```python
from langchain.embeddings import init_embeddings

embeddings = init_embeddings("openai:text-embedding-3-small")
```

## Rules

- `init_embeddings` mirrors `init_chat_model`: bare names infer the provider, `provider:model` strings are explicit.
- Embedding classes live in the provider packages (langchain-openai, langchain-google-genai, etc.), not in langchain core.
- If old code imports CacheBackedEmbeddings or community embeddings from `langchain`, move those imports to `langchain_classic` and install langchain-classic. Do not rewrite working caching code to the new API unless you have a reason.