[milo] Prepare Project entity for future expansion.
[milo] Prepare Project entity for future expansion.

By default datastore.Get(...) will refuse to load an
entity if it has unknown fields. This behavior makes it hard
to deploy changes (with potential rollbacks) that add fields
to entities.

Make Project entity tolerate unknown fields, it will be expanded
in the future CLs to contains more fields.

R=tandrii@chromium.org, e609d4c46132d2dea94acf5ac2b9f229ffc1f245@chromium.org
BUG=1090916

Change-Id: I4019e515c8b510d5c09d93139c43f73dc835c7b4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2233972
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Joey Scarr <e609d4c46132d2dea94acf5ac2b9f229ffc1f245@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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