Draft: CMake: Set SOVERSION when compiling shared libs

This patch sets a SOVERSION if shared libraries are build. This helps to integrate the library better with distributions (I am currently working on a Geant4 package for Debian). For more on concept, see https://semver.org/.

Currently I set the SOVERSION to MAJOR.MINOR, i.e. 1.1. However, I'm not entirely sure if this is aligns with how versioning policy for VecGeom. In general, the SOVERSION should be bumped if a API incompatible change has been made. Thus if e.g. 1.1.20 is allowed to have a break in the API, the SOVERSION should be MAJOR.MINOR.PATCH.