Adding support for setting expectations based on Scan filter expressions

[Szasza]

Hi @gusaul ,

I hope you are well.

Thank you for maintaining the package, it is much appreciated.

This issue is about asking if it made sense to extend Scan and ScanWithContext with a WithFilter or something similar which would behave similarly to GetItem's WithKey, just for scan filter parameters.

The relevant AWS documentation can be found at https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Scan.html#Scan.FilterExpression

If the idea is worthwhile, I am happy to contribute with a PR.