fastbootd: Implement getvar all.

This implements getvar all by invoking each callback and writing an INFO
status for each result. For commands that take arguments, the variable
handler can specify a function that returns all possible arguments.
Currently this only applies to partition variables.

Bug: 78793464
Test: fastboot getvar all works
Change-Id: I1cf84e06bf67614b6f56171c0ee6ca5d7ac383c9