[OTLP Exporter] OTLP gRPC exporter cmake build shouldn't depend on nlohmann-json

[lalitb]

Describe your environment - ubuntu, libcurl not installed, gRPC installed.

Steps to reproduce
$ cmake -DWITH_OTLP=ON ..

What is the expected behavior?
As OTLP GRPC exporter doesn't use JSON serialization, its build shouldn't try installing nlohmann-json if package is not already installed.

What did you expect to see?
Build should succeed without nlohmann-json.

What is the actual behavior?
Build tries to install nlohmann-json from submodules, or from outside if missing.

Additional context
None