Assume a CMake superproject, which uses opentelemetry-cpp in a sub directory.

The superproject builds is done in a controlled environment, such that downloading code from elsewhere is not allowed.

Any code used during the superproject build, which includes all code used by dependencies such as opentelemetry-cpp, like opentelemetry-proto, should be provided up front.

When opentelemetry-cpp builds, CMake currently attempts to:

• either use git submodules to find opentelemetry-proto
• or download opentelemetry-proto from github directly.

Please provide a way, in the opentelemetry-cpp CMakeLists.txt, to provide an explicit dependency for opentelemetry-proto.

For example, in the superproject:
OTELCPP_PROTO_PATH=/path/to/opentelemetry-proto
would cause opentelemetry-cpp to build with the code provided.