Add a fastboot command to show GSI status

The command will be used by RMA tool to skip
device wiping when it is already in GSI mode.

This change also makes it easier to add more
gsi commands in the future without needing to
update the host side tool (fastboot).

Bug: 298130522
Bug: 298138572
Test: 1. reboot into fastboot mode
      2. fastboot gsi status
Change-Id: Ic81f89a93b854f9ec70aebe2d209bfd1f98e3645