POC: Possible implementation of opentelemetry-cpp-contrib cmake project with components

[dbarker]

This is a proof of concept for adding a top level CMakeLists.txt file to this repo in order to create a versioned opentelemetry-cpp-contrib package with components.

Goals:

1. Support building a separate versioned opentelemetry-cpp-contrib package that depends on opentelmetry-cpp
2. Support two options for the opentelemetry-cpp dependency:
   • Option 1: Use find_package to import an installed opentelemetry-cpp package
   • Option 2: Fetch opentelemetry-cpp from the git repo (or git submodule) and build it together with opentelemetry-cpp-contrib in a single build tree
3. Enable creating independent components for install and import with their own third party dependencies
   • Finding like so: find_package(opentelemetry-cpp-contrib COMPONENTS exporters_geneva_metrics)

This requires some minor changes to the opentelemetry-cpp cmake install functions for general use included in this branch. https://github.com/open-telemetry/opentelemetry-cpp/compare/main...dbarker:opentelemetry-cpp:poc_otel_cmake_external_repo_support

Posting here for discussion.