API: Support for patch contexts

[CanKattwinkel]

Hello,

when working with OpenAPE I noticed that contexts can be updated using PUT but the HTTP verb PATCH is unfortunately not supported. While PUT is required for situations where you want to add / remove preferences, if you just want to update the/some preferences (e.g. a form) it would be really nice to use a PATCH method and the associated partial signature of the entity.

E.g. form is unaware of a new preference -> update via PUT deletes the new property while PATCH would respect it. That would make the API more restful.

best regards,
can