[OTLP EXPORTERS] add otlp_common target for shared otlp utils

[dbarker]

Contributes to #4239 and #4249

Adds an otlp_common target/library to hold the common utilities (just otlp_envornment.<h,cc> for now).

Changes

• update cmake and bazel build scripts to create opentelemetry-cpp::otlp_common
• update the docs to add the new target.
• update the cmake install test to cover the new target.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.87%. Comparing base (89729e1) to head (a1e4c21).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4333      +/-   ##
==========================================
+ Coverage   80.86%   80.87%   +0.02%     
==========================================
  Files         450      450              
  Lines       19216    19216              
==========================================
+ Hits        15537    15539       +2     
+ Misses       3679     3677       -2     

see 1 file with indirect coverage changes

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