Feature description

By default the env endpoint returns a JSON Object such as:

{
"activeEnvironments": ...
"packages": 
"propertySources": 
}

I would like to be able to return just:

{
"activeEnvironments": ...
}

For example to be able to know which environments my application is using without having to check the logs.

It would be good to be able to have configuration property endpoints.env.keys with a List of Strings wit the default value ['activeEnvironments','packages','propertySources']

Users will be able to expose only `activeEnvironments by setting:

endpoints.env.keys[0]=activeEnvironments