Remove the perspective feature

[claustres]

Perspectives are used on services to identify nested object properties that should not be retrieved by default, if you'd like to retrieve it you have to use $select. However, this complexifies the way editors are implemented when targetting a perspective. Moreover, it does not work well with some feathers modules like feathersjs-ecosystem/feathers-reactive#214.

We should try to remove it in KDK to let applications chooose wich properties to hide using discard() or preventChanges() hooks. We should however still allow editors to work on nested properties.

[claustres]

For now we only keep the ability to edit a nested object using an editor, all other related perspective things are gone.