[realms] Add roles and permissions related to service accounts.
[realms] Add roles and permissions related to service accounts.

R=tandrii@chromium.org, jwata@google.com
BUG=1082960

Change-Id: Iad27222c9611cc320814f50d3556d9ea70509634
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2211080
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py