Separate configuration for management UI, REST APIs, and metrics

[NiccoMlt]

Currently, REST APIs, management UI built upon them, and metrics are on differente servlets of the same Jetty server, but are all started by the configuration http.admin.enabled; also one or both http.admin.port and https.admin.port must be set for the server to correctly be available.

Each interface should have its own setting, like:

• http.admin.ui.enabled to enable Vue management UI and REST APIs (which are needed by the UI itself);
• http.admin.api.enabled to enable only the REST APIs;
• http.admin.metrics.enabled to enable the metrics APIs;
• keep the legacy http.admin.enabled to enable everything on the admin Jetty server.

We should also consider if each feature can keep sharing the same HTTP and/or HTTPS port(s) (and potentially SSL certificate) or if we should split those options too.

[NiccoMlt]

We should also consider if each feature can keep sharing the same HTTP and/or HTTPS port(s) (and potentially SSL certificate) or if we should split those options too.

@aluccaroni @hamadodene @dmercuriali what do you think about this?