Design of building OpenTelemetry C++ SDK and exporters as DLL

NOTE: the current build produces a single DLL which includes API, SDK and exporters. This MAY change in future.

OpenTelemetry C++ can be built into single DLL named opentelemetry_cpp.dll on Windows, with CMake flag OPENTELEMETRY_BUILD_DLL set to ON.

To instrument an application based on the DLL, please define macro OPENTELEMETRY_BUILD_IMPORT_DLL before including any OpenTelemetry API header files in the application source files.