Extension WebHook provider doesn't support local

Currently our IWebHookProvider implementation by default returns null when running locally (code [here](https://github.com/Azure/azure-webjobs-sdk-script/blob/5e04b953944e17e965a33e83173d92c6ba5993ca/src/WebJobs.Script.WebHost/WebHooks/WebJobsSdkExtensionHookProvider.cs#L57)). The code appears to have supported running locally at one time, as evidenced by the "localhost:" references in that method. However, some short cut logic seems to have been put in place based on the WEBSITE_HOSTNAME app setting (which won't be set locally).

We should update this method so it works when running locally as well. We can use the existing `ScriptSettingsManager.IsAzureEnvironment` that we use elsewhere. Then we just need to get the right local host name.