[identity] Add function to remove user devices' data

Summary:
Added a function that prunes all user's devices data, while keeping device lists.
Removes all DeviceRow DDB items for given user_id. It does this by querying items with specific sort key prefix.

Made it return list of removed device IDs, it will be used for Tunnelbroker to remove their data too.

Depends on D12587

Test Plan: Cargo check. The logic is tested later in the stack

Reviewers: kamil, varun, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12588