I am reading about the HTTP Binding, which seems I should configure a HTTP request for every Item.
Is there a way to make openHAB automatically send a HTTP request every time when an Item received a Command or an Update?
Why? Please describe what you are trying to achieve. There is almost certainly a better approach.
For a test project I want to just use openHAB as the “man-in-the-middle” to connect a few Smart Home systems (Hue, Sonos, Z-Wave etc.), but do the actual logic in another framework, where I can program (and test) faster than the openHAB Java code via rules files. I am not very efficient with that.
I would suggest looking into MQTT and the MQTT Event Bus configuration as the most painless approach.
Beyond that, going down an HTTP Binding approach will require you to do so much work and coding in OH Rules that you may as well code your project in the Rules themselves and bypass the other framework entirely.
Are you aware of the JSR223 and Experimental Rules engine? It lets you code Rules in JavaScript, Jython, or Groovy rather than the Xtend based Rules Domain Specific Language.