[🚀] REST-endpoint for system availability state

[MarkL4YG]

Describe the solution you'd like
A standard REST endpoint should be added to the framework to indicate the framework is online and REST requests can be served.
It should return at least the current version of the framework.
Other information that may be useful is open for discussion (@finnWellers )

Describe why you would like to see this implemented
This way, applications can check for availability with a small request before starting real requests.

[finnWellers]

It should be relatively easy to implement by registering a system Controller. Maybe we should think if we want to treat system controllers differently than plugin controllers

[MarkL4YG]

We could theoretically do that but at the moment I don't think any special treatment is necessary.

[finnWellers]

This should be a prerequisite for #12

[finnWellers]

Also add information about registered Controllers and Plugins