[Event Grid Trigger] Validation handshake needs to check WebHook-Request-Origin when sending WebHook-Allowed-Origin response header

[JoshLove-msft]

We currently hardcode the public cloud endpoint - https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/eventgrid/Microsoft.Azure.WebJobs.Extensions.EventGrid/src/TriggerBinding/HttpRequestProcessor.cs#L54. This won't work if the function app is in gov cloud.

[JoshLove-msft]

Note we should still only reply with endpoints that we know about, e.g. gov, cn, us clouds.