emul: Add SBS Gauge emulator backend_api

In order to support easier setup of test scenarios with fuel gauge
emulators, we should expose an API that can change internal emulator state.

Add a minimal fuel gauge emulator backend API for setting the charging
current and voltage with a sample implementation in the emul_sbs_gauge with
an associated driver test.

Signed-off-by: Aaron Massey <aaronmassey@google.com>