[services-lib] Add integer and timestamp parsing helpers

Summary:
Adds a helper function to parse UTC timestamp DynamoDB attribute values.
Also improved the existing number parsing function to be more flexible
and renamed it to parse_integer because it was created specifically for integers

Test Plan: Unit tests

Reviewers: michal, patryk, jon, varun

Reviewed By: michal, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8438