[identity] replace cache calls with DB client method calls

Summary:

• replace all inserts into cache with inserts into DDB table
• replace all gets from cache with gets from DDB table
• remove all references to moka from crate

DDB TTL will handle removing old items from the table.

Depends on D11255

Test Plan: tested that password login, registration, and update continue to work as expected on staging. verified that items were eventually automatically removed from the table

Reviewers: will, bartek, michal

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11256