[C++] Use find_package insead of adding include directories 1 by 1

[svrnm]

via #3423 (comment) from @ThomsonTan

Could here use find_package(opentelemetry-cpp CONFIG) instead of adding include directories 1 by 1?

cc @Akhaled19

[svrnm]

Tagged this as good first issue and help wanted. Note that this is only a good first issue if someone knows C++ well enough to tackle it (since this is required, not so much being able to work with our docs)

[ricardoamaro]

Giving a try on this one

[ricardoamaro]

Here is my try #4309