Add endpoints for complete management of embeddings

[prantlf]

Based on #6.

• GET /collections - list collection names
• GET /collections/:collection_name/embeddings - get embedding identifiers
• POST /collections/:collection_name/embeddings - filter embeddings with metadata
• DELETE /collections/:collection_name/embeddings - delete embeddings by metadata
• GET /collections/:collection_name/embeddings/:embedding_id - get an embedding
• DELETE /collections/:collection_name/embeddings/:embedding_id - delete an embedding

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• [ ] Apply: All new business logic should have corresponding unit tests.
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