HTTP-interface for radioactivity_add_energy

Suppose that we'd like to report radioactivity events from an external server (see #279456: Plugin: Support for arbitrary (external) targets). Probably the simplest way would be to make radioactivity_add_energy() invokable by HTTP requests, e.g. http://some-server/radioactivity/add_energy?id=object_id&class=object_class&source=some_source

However, we need some extra measures to make this secure, and I'm thinking of the following:

• We could limit the access at the web server (e.g. apache) or reverse proxy (e.g. squid) level. Probably that's enough for IP-based security.
• We could add signatures to the requests, like extra query parameter, e.g. hash=md5($id . $class . $source . $private_key). That should keep the casual malevolent internet entity from tampering our data, in case the IP-based access restriction is impractical to implement