[Identity] Add a database client

Summary:
Introduces a DynamoDB client and implements a method to fetch PAKE registration data from a DDB table.

Depends on D3852

Test Plan: Was able to fetch data from a test table in DynamoDB

Reviewers: jimpo, karol-bisztyga, geekbrother, palys-swm, ashoat

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

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3853