Reduce PartitionKey allocations

The current design of the API leads to 2 extra objects being allocated for every call that takes a partition key when using a non-string value:

1. A new instance of PartitionKey.
2. Boxing of the value.

One way of avoiding these would be by making PartitionKey a generic struct. Alternatively the overloads that take a string instead of PartitionKey could be added back.

[kirankumarkolli]

@AndriySvyryd thanks for the feedback.

PartitionKey as struct makes sense, it enables non–breaking optimization later. Current cosmos wire contract is we could only avoid one object heap allocation.

Generic struct vs overloaded CTOR: Usability is one concern that needs attention for generic structs. We could explore using generic’s as optimization underneath with overloaded CTOR as contract.

[AndriySvyryd]

Should I open an issue to track further optimizations or you are tracking them elsewhere?