[CONFIGURATION/BUILD] Resource detector targets and README

[dbarker]

Contributes to: #3548
Follow up to #4417 (comment)

The spec defines built-in resource detectors as host, service, process, and container along with an experimental EnvEntity detector.

The detectors should follow the Entity semantic conventions for attributes:

• Entity: Host
• Entity: Process
• Entity: Service
• Entity: Container

The implemented resource detectors support a small subset of these attributes and are currently built into a single target (and single test executable).

This PR sets up the documentation and creates individual targets for each detector that will make it easier for contributors to add support for attributes. Individual detector targets will allow users to link only what they use.

Changes

• Create individual targets (CMake and Bazel) for each detector and builder
• Create two interface targets to group all detector and builders respectively
• Update builders to link detector targets privately
• Break up the one unit test exe into per detector/builder tests
• Update install tests to use the new targets
• Add README for the detectors directory

Please provide a brief description of the changes here.

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 82.58%. Comparing base (3fb1d31) to head (0836842).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4430   +/-   ##
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  Coverage   82.58%   82.58%           
=======================================
  Files         511      511           
  Lines       20039    20039           
=======================================
  Hits        16548    16548           
  Misses       3491     3491           

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[marcalff]

LGTM