fastbootd: Add command to remove GSI installs

Note: this only removes the bootable marker, since we're unable to
remove the userdata files within recovery.

Bug: 121210348
Test: fastboot gsi wipe
      fastboot gsi disable
Change-Id: I64fe848c787d426ae9d18a1557a9d6b340bfc2cf
diff --git a/fastboot/device/commands.h b/fastboot/device/commands.h
index bb1f988..afd6d08 100644
--- a/fastboot/device/commands.h
+++ b/fastboot/device/commands.h
@@ -48,3 +48,4 @@
 bool ResizePartitionHandler(FastbootDevice* device, const std::vector<std::string>& args);
 bool UpdateSuperHandler(FastbootDevice* device, const std::vector<std::string>& args);
 bool OemCmdHandler(FastbootDevice* device, const std::vector<std::string>& args);
+bool GsiHandler(FastbootDevice* device, const std::vector<std::string>& args);