Allow partition key to be passed as a parameter

[adrianlungu]

Hello,

Looking at the FixedWindowDynamoDB (and probably the others as well), it would be great if the partitionKey could be passed as a parameter in Increment instead of as part of the struct.

In highly concurrent scenarios, where multiple keys would need to be handled potentially at the same time, being able to re-use the same FixedWindow per table, and not per key, to increase multiple keys via the function call would be ideal.

I didn't get a chance to look at the extent of how such a change would impact the library as a whole, for FixedWindowDynamoDB it does not look like a big change, although it would be a breaking change.