Gateway gets out of sync when Upstreams change

[ilackarms]

Describe the bug

When a VS with a function route references an upstream that doesn't yet have the function, that VS will not be written in the proxy.

When the upstream is updated (for example by FDS), Gateway has no knowledge to resync, and the VS is never added to the proxy (until another event causes a resync).

To Reproduce

Create a Swagger or GRPC Deployment/Service and a VS with a function route to the expected upstream at the same time.

The VS will get an error reported to it, and the VS will not be included in the Proxy, even after FDS discovers the functions.

Expected behavior

If the US gets updated, the VS will become accepted.