Allow Framework and Go4Engine to be built separately.

[mvonbuelow]

This PR allows to build the GO4 Plugin and the Framework separately. This allows to model dependencies more flexibile. The NIX flake will then have two Deriviations: stream.framework and stream.go4engine. Only stream.go4engine will depend on Go4.

---

• Use -DENABLE_FRAMEWORK=OFF -DENABLE_GO4ENGINE=ON to build only go4engine
• Use -DENABLE_FRAMEWORK=ON -DENABLE_GO4ENGINE=OFF to build only framework
• default is both ON

---

• create alias targets stream::Stream and stream::StreamDict
• use find_package(Stream) in go4engine to make sure that it can build separately (if it's build internally, then the aliases apply)