Base URL is incorrect when using with proxy

**Describe the issue**
After enabling the proxies on V4 and adding the below proxy:
```
{
"$schema": "http://json.schemastore.org/proxies",
"proxies": {
"swagger": {
"matchCondition": {
"methods": [ "GET" ],
"route": "/"
},
"backendUri": "https://localhost/swagger/ui"
}
}
}
```
and opening the function from Visual Studio
`http://localhost:7071/swagger/ui` this works fine
`http://localhost:7071/` shows CORS errors:

After a quick investigation, I noticed that the URL passed to SwaggerUIBundle is wrong:

Here it is correct:
and when opening the root it is wrong:

Probably calculated here: https://github.com/Azure/azure-functions-openapi-extension/blob/main/src/Microsoft.Azure.WebJobs.Extensions.OpenApi.Core/SwaggerUI.cs#L61

I'll try to download the source code and debug it locally.

Start at src/Microsoft.Azure.WebJobs.Extensions.OpenApi.Core/SwaggerUI.cs around line 61 and reproduce the proxy configuration with the root URL and /swagger/ui URL in Visual Studio. Compare the URL passed to SwaggerUIBundle in both cases; done means the root route produces the correct base URL and no CORS errors.

Written by the indexing model from the issue text.