Add support for customizing Swagger UI URL

**Describe the issue**
Currently, the URL of Swagger UI is hardcoded to `swagger/ui` (https://github.com/Azure/azure-functions-openapi-extension/blob/main/src/Microsoft.Azure.WebJobs.Extensions.OpenApi/OpenApiTriggerFunctionProvider.cs#L91).
Ideally, we should be able to customize the URL.

By default, after publishing the function, the swagger UI is available at: https://testaccount.azurewebsites.net/api/swagger/ui

we can remove the `api` prefix by adding the below lines to `host.json`:
```
"extensions": {
"http": {
"routePrefix": ""
}
},
```

but with the above lines, the URL will look like this: https://testaccount.azurewebsites.net/swagger/ui

Ideally, we should be able to serve Swagger UI from the root, that is: https://testaccount.azurewebsites.net

Start with OpenApiTriggerFunctionProvider.cs at the hardcoded swagger/ui route mentioned in the issue. Trace how HTTP routes and host.json settings are applied, then determine where a configurable UI path belongs. Done means the Swagger UI can be served at a customized path, including the application root, without breaking the default route.

Written by the indexing model from the issue text.