Support for X-Forwarded-Host

[mathiasi]

Describe the issue
We have Function Apps that are accessed through a Front Door. This gives us some trouble with the URL in the swagger UI as the URL is being resolved to the direct URL of the Function App which is not what we want as it is not accessible and everything should go through the Front Door. Is there a way to configure the URL being set in the Swagger UI? As far as I can tell this is the logic that sets it and it doesn't appear there is a way to change it:

azure-functions-openapi-extension/src/Microsoft.Azure.WebJobs.Extensions.OpenApi.Core/SwaggerUI.cs

Line 61 in ab184cb

var baseUrl = $"{this._req.GetScheme(options)}://{this._req.Host}{prefix}";

I know that the Front Door will send X-Forwarded-Host and X-Forwarded-Proto which we were able to use with older the older Swashbuckle library which we used for In-Process Function Apps.

To Reproduce
Steps to reproduce the behavior:

1. Deploy Function App
2. Configure Front Door
3. Try to access Swagger UI through Front Door
4. Observe non-Front Door URL being used

Expected behavior
Front Door URL should be used

Screenshots

[mathiasi]

I realize that it is actually possible to change the value by manually specifying the server using the configuration. I'll leave this open for somebody else to judge if it is a good idea to support the header anyway. But I found a "workaround".