API: allow plugins to extend existing API models

Like title says.
Being able to alter the “model” (the json shape an object gets transformed into when requested via the REST api) would allow plugin developers to add “custom properties” to things that currently don’t allow them.

For example file blueprints could have plugin specific properties that would then be loaded by the panel via the api/pages/(page)/files/(file)?view=panel request and a plugin altering the FileView could react to those properties.

related issue with some more info: https://github.com/getkirby/ideas/issues/275