Add a command to get the current synced version from cros-sdk directory about cro3

[HidenoriKobayashi]

I'd like to try this one, but I need clarifications.

The following is an example output for brya on my local workstation

 gs://chromeos-prebuilt/board/amd64-generic/postsubmit-R116-15505.0.0-52093-8778090689467554065/packages gs://chromeos-prebuilt/board/amd64-generic/postsubmit-R116-15505.0.0-52094-8778089119028579249/packages gs://chromeos-prebuilt/board/amd64-generic/postsubmit-R116-15505.0.0-52095-8778086878663453457/packages gs://chromeos-prebuilt/board/amd64-generic/postsubmit-R116-15505.0.0-52096-8778085176293458977/packages gs://chromeos-prebuilt/board/brya/postsubmit-R116-15505.0.0-83679-8778090996123150033/packages gs://chromeos-prebuilt/board/brya/postsubmit-R116-15505.0.0-83680-8778090645377985841/packages gs://chromeos-prebuilt/board/brya/postsubmit-R116-15505.0.0-83681-8778089074903851649/packages gs://chromeos-prebuilt/board/brya/postsubmit-R116-15505.0.0-83682-8778086834597103329/packages gs://chromeos-prebuilt/board/amd64-generic/cq-R116-15502.0.0-52012-8778246588928878481/packages

• Can I just pick the version string (R116-15505.0.0) from the first entry?
• How does the CLI look like? (under sync? or a new command like repo?)

[hikalium]

@mhiramat I found that I filed the bug already, so please close this once the implementation is done!!!!