Support filtering with metadata

[prantlf]

Add a simple metadata filter based on equality of key-value pairs.

• An undefined filter or an empty array matches all embeddings.
• An empty object in the array matches any embedding.
• If an object in the array is not empty, it will match an embedding, if all its key-value pairs are found in the embedding's metadata.
• If there are more than one objects in the array, the whole filter will match an embedding, if at least one object matches the embedding's metadata.

Example:

[ { "asset": "book", "kind": "paperback" }, { "asset": "newspaper" } ]

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