[Identity] DatabaseClient method to put pake registration data

Summary:
The client will use this method to put PAKE registration data in our DynamoDB table. We serialize the ServerRegistration object and store it as bytes.

Depends on D3959

Test Plan: Successfully put an item in the PAKE registration table in DynamoDB

Reviewers: jimpo, karol-bisztyga, palys-swm

Reviewed By: jimpo, karol-bisztyga

Subscribers: ashoat, Adrian, yayabosh, atul

Differential Revision: https://phabricator.ashoat.com/D3960