Split extension webhook handler registration and URL retrieval

Currently, the call to get the extension webhook route/URL handles the handler registration:
https://github.com/Azure/azure-webjobs-sdk-script/blob/8309e2b58008d8abed55d419f3e7e23d771f7ede/src/WebJobs.Script.WebHost/WebHooks/WebJobsSdkExtensionHookProvider.cs#L37-L50

There's no way for extensions to perform registration without triggering the code path that will result in a key generation (there's also no way to just get the URL without performing a handler registration).

Extensions like the EventGrid trigger do not need to get the URL at initialization, so splitting this call into two distinct APIs would avoid a potentially expensive operation and a persisted key that may never be used/needed.