[swarming] Implement skeleton to register MISSING BotEvent
[swarming] Implement skeleton to register MISSING BotEvent

No functional change yet.

Bug: 916578
Change-Id: I264c58ce001aaa6f412d31234dc285366e6f6f52
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1884464
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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