Create default host.json on function app creation from portal

Function host does not have a way to differentiate between a new function app and existing function app. We need to add a default config to host.json on when function app is created via portal. This cannot be done from function host since we run the risk of adding config to existing customers as well.

The issue does not name files, tests, or an entry point. Start by tracing function app creation in the portal and how host.json is provisioned, then identify the portal-side location where a default configuration can be added without affecting existing apps. Done means newly created portal apps receive the default host.json configuration while existing apps remain unchanged.

Written by the indexing model from the issue text.