cache server: add missing endpoints

Update cache server to implement the static, stage, is_staged, and
check_health endpoints

BUG=b:272075787
TEST=privision device and run `tast -verbose=true -logtime=false run
-sshretries=2 -downloaddata=lazy -totalshards=1 -shardindex=0
-ephemeraldevserver=false -build=false -downloadprivatebundles=true
'-devservers=http://localhost:3335' 'localhost:2222' 'example.Pass'`

Change-Id: I924d748d63cbf597add4020dee75b063445febda
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4362898
Tested-by: Jack Gelinas <jackgelinas@google.com>
Commit-Queue: Jack Gelinas <jackgelinas@google.com>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
1 file changed