Changes -
* Add CMake build support to `grpcpp_otel_plugin`. Currently, we are only supporting the `find_package CONFIG` method for depending on `opentelemetry-cpp`.
* Since, `grpcpp_otel_plugin` is an extension of gRPC, it will not be built by default. To enable building of this target, a new CMake option `gRPC_BUILD_GRPCPP_OTEL_PLUGIN` is being added.
* Also add `CMakeLists.txt` to the otel example.
The `otel_plugin_test` can also be built through cmake but, for the CI to work, there are some additional changes. Those will be made in an upcoming PR.
Closes#36063
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36063 from yashykt:OTelCMakeSupport 3bc783823b
PiperOrigin-RevId: 613734473
Yash Tibrewal