[Identity] Remove static lifetime constraint on DBItemError

Summary:
In order to support device maps, we can't assume
that all attribute keys will be known staticly.

In support of https://linear.app/comm/issue/ENG-4445

Depends on D8607

Test Plan:

cd identity && cargo build

or run any integration tests

Reviewers: bartek, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8627