Cosmos: Allow creating a `PartitionKey` from a `Vec`

[analogrelay]

Currently, a PartitionKey can be created from a single item of a supported type (&str, numbers, etc.) or a tuple of 2 or 3 items of a supported type.

However, as I'm using the SDK a bit, I think it would be useful to support creating one from a Vec<T> (where T is something that can be converted to a partition key value). So, we should add an implementation of From<Vec<PartitionKeyValue>> for PartitionKey.