Added the `Yarp.ReverseProxy` package and configured the app to use
YARP for forwarding specific authentication-related API requests
to an external service (`auth-hub`). Updated `Program.cs` to load
YARP configuration from a new `yarp.json` file and added middleware
to map unmatched requests to the reverse proxy.
Replaced old routes and clusters with new routes (`auth-login`,
`auth-envelope-receiver-login`) and a new cluster (`auth-hub`)
pointing to `https://localhost:9090`. Configured route
transformations for path and query parameter adjustments.
These changes improve modularity and scalability by enabling
dynamic reverse proxy configuration and external service
integration.