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
diff --git a/fastboot/device/utility.h b/fastboot/device/utility.h
index 0931fc3..4f0d079 100644
--- a/fastboot/device/utility.h
+++ b/fastboot/device/utility.h
@@ -56,5 +56,5 @@
 bool LogicalPartitionExists(const std::string& name, const std::string& slot_suffix,
                             bool* is_zero_length = nullptr);
 bool OpenPartition(FastbootDevice* device, const std::string& name, PartitionHandle* handle);
-
 bool GetSlotNumber(const std::string& slot, android::hardware::boot::V1_0::Slot* number);
+std::vector<std::string> ListPartitions(FastbootDevice* device);