[resultdb] Mark Invocation.created_by as OUTPUT_ONLY
[resultdb] Mark Invocation.created_by as OUTPUT_ONLY

The value of this field is populated by ResultDB.

R=1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org

Change-Id: Ia58aa5273dc5527c636ec491021a8e21bf704925
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2158088
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

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