[gateway][cli][controlplane][api] Improvements on startup and new GET Application API without stats

[eolivelli]

Summary:

• currently the "langstream gateway chat" command is very slow at bootstrap
• one reason is that it gets twice the status of all the pods, this happens because it tries to validate the gateway ids, and it issues a GET /tenant/application that currently returns the full status of the pods

Changes:

• change the get application endpoint (/tenant/application) to return only the description of the application by default
• there is a new parameter ?stats=true|false. If you set stats=true then you get the whole application description + metrics + status information

This is a small breaking change in the REST API:
in order to get full metrics you have to add stats=true to the API call
if we already cover the API in the docs, then we have to update it