[services][feature-flags] Introduce constants for accessing DynamoDB table

Summary:
Add constants that will be used to read the values from DynamoDB.

Depends on D6687

Test Plan: Tested with the rest of the stack. Check if there are no warnings.

Reviewers: michal, bartek, jon, varun

Reviewed By: bartek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6852