Building OTLP exporter from the release tarball fails for opentelemetry-proto dependency

[lalitb]

Describe your environment
ubuntu 20.04

Steps to reproduce
Download the release tarball, and try building OTLP exporter with cmake option WITH_OTLP=ON
The problem is that the release tarball contains the third_party/opentelemetry-proto directory but it is empty as it is a git submodule and the CI/build is not including this as part of the tarball).

What is the expected behavior?
Build to be successful.

What is the actual behavior?
Build fails

Additional context
Few ways I can think to fix it:

1 Let cmake download the opentelemetry-proto dependency at build time using (say) FetchContent if the third_party/opentelemetry-proto directory is empty. FetchContent is introduced in cmake 3.14, so we may have to upgrade the cmake version for this.
2. Pre-compile proto source(cpp) and headers (hpp) using protocol buffer compiler and check-in them in the repo ( similar to how opentelemetry-python does it - https://github.com/open-telemetry/opentelemetry-python/tree/main/opentelemetry-proto ). We can also have a script to generate these files, to be used every time the version needs to be bumped.

[lalitb]

@esigo - fyi, have assigned it to you as discussed in today's meeting.

Seems I am not able to assign it to you (can't find your name from the search bar), please feel free to pick it up.

[esigo]

@lalitb seems I can't assign it to myself. I will see if I can come up with a solution that doesn't require cmake version change.

[esigo]

I will use ExternalProject_Add as it is available in cmake 3.1.

[esigo]

There is a similar issue with nlohmann_json. I try to fix both.