battery-service: Re-export `embassy_time::Duration`

[bjackson312006]

Description

The embassy_time::Duration type is exposed in battery-service's public API via get_timeout() and set_timeout() from controller.rs, but is not re-exported. This creates annoying dependency issues where, to implement the Controller trait, users of battery-service need to pull embassy-time into their project and hardcode it at the exact version battery-service uses.

Proposed Solution

In either controller.rs or lib.rs, add a pub use embassy_time::Duration; to re-export the type.