refactor(unset): remove the AstarteType::Unset

[joshuachp]

Move the Unset to the Aggregation enum and introduce a new unset method. This simplify the checks since you can only unset a property and the AstarteType::Unset is not needed in the others code paths. Also this simplifies the conversions in the gRPC code that now in infallible.

[codecov[bot]]

Codecov Report

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[joshuachp]

I propose the Value for the name of the Aggregate, this is the same name in the MQTT protocol (the v field of the bson document).

This is because we are already "over-using" payload as the name of the body of a message/request in the transport structs.