Ergonomic Payload::deserialize (unoptimised version)

[TmLev]

Continuation of #37, #37 (comment), and #37 (comment).

This PR enables library users to utilize the straightforward method Payload::deserialize(self) to deserialize a payload into any type T, as long as T implements serde::de::DeserializeOwned.

The current approach carries significant overhead as it initially serializes a payload into serde_json::Value and then deserializes it back.

One possible solution to mitigate this overhead could be the use of MapDeserializer (see more). However, this would require implementing IntoDeserializer for qdrant::Value, something which I was unable to accomplish.