[server] Add a tool for generating Tink AES keys stored in GSM.
[server] Add a tool for generating Tink AES keys stored in GSM.

It will be used to bootstrap and rotate keys used to
encrypt cookies.

R=tandrii@chromium.org
BUG=712506

Change-Id: I546208f4c2dee444bb16e5a47fc37a093af164c7
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2802034
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go