Implementation of install target in CMake

[smartnet-club]

What about installation target in cmake?

Similar to https://github.com/grpc/grpc/blob/master/CMakeLists.txt

if (gRPC_INSTALL) install(EXPORT gRPCTargets DESTINATION ${gRPC_INSTALL_CMAKEDIR} NAMESPACE gRPC:: ) endif()

[g-easy]

This is one of the bullet points tracked in #244.

[g-easy]

abseil/abseil-cpp@bf29470 - This somewhat large commit contains abseil's implementation of cmake install. We should take this into consideration when we do this for OpenCensus.