[services][identity] helper function to create primary key for DynamoDB

Summary:
This function takes a partition key and optional sort key and returns a primary key for the DynamoDB client.

Depends on D3920

Test Plan: Unit tested

Reviewers: jimpo, karol-bisztyga, palys-swm

Reviewed By: jimpo, karol-bisztyga, palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4015