[services][identity] helper function to parse map attributes from dynamodb

Summary:
fairly straightforward. we take an Option<AttributeValue> that should hold a HashMap and then return the HashMap or an error if it's the wrong type or missing

Depends on D5567

Test Plan: this helper function is tested two diffs later when the getUserPublicKey RPC is actual callable

Reviewers: tomek, bartek, jon

Reviewed By: tomek, bartek, jon

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5568