[identity] Add device list signatures to DDB

Summary:
Added signature fields to DeviceListRow struct and updated conversion traits to read/write corresponding DDB attributes.
Business logic is going to be connected in a later diff

Depends on D11896

Test Plan: Mocked signature values and made sure they're correctly stored in DDB (awslocal dynamodb scan)

Reviewers: kamil, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11897