Fix dependencies when building core as shared libs

[JonasVautherin]

When building the core as shared libraries, the dependencies (e.g. curl) are linked statically and therefore are not required to be linked further.

This PR fixes the build system such that those dependencies are linked using PRIVATE, meaning that they don't propagate to DronecodeSDKTargets.cmake.

Resolves #758.

[JonasVautherin]

@julianoes: So I set BUILD_SHARED_LIBS=ON by default, because it seems like it is what people want to do usually (and then install that). It also makes sense for the backend. In our case, we tend to link our libs statically against the backend, so the CI gets -DBUILD_SHARED_LIBS=OFF when -DBUILD_BACKEND=ON, but that's our choice. Does that make sense to you?

[julianoes]

set BUILD_SHARED_LIBS=ON by default

Yes I think that makes sense.

[JonasVautherin]

Always surprised to see that the coverage can decrease by 3.4% with such changes 😅