[CMake] Add option to specify a dependency is test only

[filippobrizzi]

Description

To improve modularity and reduce dependencies, I added an option to specify that a module is a test dependency only.
In this way, when building with BUILD_TESTING=OFF we can speed things up and reduce coupling.

To showcase the improvement I updated the unit test of the bag module:

• Remove the custom proto and instead use dummy_type
• Make types_proto and random test only dependency

[floriantschopp]

niice