Access IIS SiteName at configuration time (without HttpContext)

### Is your feature request related to a problem? Please describe.
I need to access the IIS SiteName at configuration time to setup some paths that need to be unique by site (I instantiate the same app several times under different site names in IIS).

The current API (see #17069) only give access to it through server variables, which are only accessible on an HttpContext.

### Describe the solution you'd like
The dotnet fx API provides HostingEnvironment.SiteName which allowed me to access it at application startup. I guess the new API would be under the form of a new Feature in IApplicationBuilder.ServerFeatures but I have no idea how to implement it.

### Additional context
If the request is too niche to be considered, I would appreciate any ideas/pointers on how I could implement it myself.