BYOB Web hook configuration should not depend on the existence of WEBSITE_HOSTNAME

The current implementation of web hooks in BYOB depends on the `WEBSITE_HOSTNAME` app setting being present. This setting always exists in Azure, but not when running locally. This causes problems for locally hosted apps because a static app setting needs to be configured, and it could be completely different from the value actually used by the local web server. This caused multiple headaches during the development and testing of Durable Functions.

I don't have a suggestion for the right way to fix this right now, except to say that web hook configuration may need to be more tightly coupled to the web host. Alternatively, we could look into relying on a request message to get any host information we need - e.g. for computing redirect URLs (one workaround I've implemented in my extension is to ignore the scheme and host part of the web hook URL and instead get it from the incoming HTTP request when generating a redirect to a web hook URL).

No files or tests are named. Start by tracing BYOB webhook configuration and every use of the WEBSITE_HOSTNAME app setting, then inspect how the web host and incoming HTTP request expose host information. Done should mean locally hosted apps no longer require a static WEBSITE_HOSTNAME value and webhook redirects use the correct host.

Written by the indexing model from the issue text.