Add subscribers for AppConnector events
Make the RouteAdvertiser interface optional We cannot yet remove it because
the tests still depend on it to verify correctness. We will need to separately
update the test fixtures to remove that dependency.
Publish RouteInfo via the event bus, so we do not need a callback to do that.
Replace it with a flag that indicates whether to treat the route info the connector
has as "definitive" for filtering purposes.
Update the tests to simplify the construction of AppConnector values now that a
store callback is no longer required. Also fix a couple of pre-existing racy tests that
were hidden by not being concurrent in the same way production is.
Updates #15160
Updates #17192
Change-Id: Id39525c0f02184e88feaf0d8a3c05504850e47ee
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>