Resolve discrepancy in Http/WebHook multi-key scenarios

In WebHook multi-key scenarios, to specify a key other than default, a **clientid** query parameter is required:

`https://myfunctions.azurewebsites.net/api/mywebhook?clientid=c1&code=xyz123`

Whereas for vanilla http functions no secondary param is required to identify the key. The reason for this is historical, because WebHook auth is built on the ASP.NET WebHooks library which requires this model, and that integration came after our http function model.

We should investigate ways of smoothing over this discrepancy.

Start by reproducing the documented WebHook and vanilla HTTP multi-key request examples and compare how each identifies a non-default key. Trace the existing authentication handling for both paths. Done means the discrepancy is resolved without breaking either request model, with behavior covered by appropriate tests.

Written by the indexing model from the issue text.