at_commons: Deprecate the obsolete "isHidden" attribute from Metadata

[gkc]

Is your feature request related to a problem? Please describe.

"isHidden" should be removed from metadata; the "hidden" semantic is achieved only by convention in the atServer and in the atClient's LocalSecondary, by prefixing a key with 1 or 2 underscores as per example below. isHidden is neither stored nor referenced by the atServer, nor by the atClient's LocalSecondary

• // public hidden key: public:__location@alice 
  

• // self hidden key: _location@alice
  

Describe the solution you'd like

Mark isHidden as Deprecated in at_commons metadata. Remove references to isHidden from all packages which depend on at_commons - I have found references in at_client, at_lookup and at_login_flutter but there may be more. Note that there is actually code in AtLookupImpl's update method which silently prefixes an underscore if isHidden is set to true; we need to check if any app or example or widget uses this feature