Handle "before" annotations on REST interface classes.

[s-ludwig]

Fixes #1753.

[Geod24]

Shouldn't @before on interface be discouraged ? The problem is that it describes server-only behavior so it can easily make the interface unusable for client generation (e.g. if the @before function have dependency on the DB, it drags it into the client as well).

[s-ludwig]

Definitely should be discouraged in their current form. The problem is that only those on the interface seem to have worked for a while now and I didn't notice when that became the case (don't use them anywhere anymore, AFAIR). So I'd let them go through a full deprecation cycle (doc -> warn -> error) to make sure they only appear on the class (and maybe any base class).