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).

(Cherry-picked from aosp/2735077)

Bug: 298533290
Bug: 298138572
Test: 1. reboot into fastboot mode
      2. fastboot gsi status
Merged-In: Ic81f89a93b854f9ec70aebe2d209bfd1f98e3645
Change-Id: Ic81f89a93b854f9ec70aebe2d209bfd1f98e3645