gsi_util: adding flash_gsi subcommand

This change implements the flash_gsi subcommand:
  1. Erase userdata/cache partition
  2. Erase metadata partition (optional)
  3. Flash generic system image
  4. Disable Android Verified Boot (optional)
  5. Reboot the device

Bug: 70253882
Test: make gsi_util
Test: gsi_util flash_gsi
Test: gsi_util --debug flash_gsi --image system.img
Change-Id: I552c04aaf227c6fca2e5c3caab26d713e2f6f5d0