[CMAKE] Switch opentelemetry-proto to use FetchContent

[dbarker]

Fixes # (issue)

Changes

• Switch opentelemetry-proto to use the FetchContent module instead of the ExternalProject module.
• Set and log the opentelemetry-proto_VERSION and opentelemetry-proto_PROVIDER variables.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	4c9676c
🔍 Latest deploy log	https://app.netlify.com/projects/opentelemetry-cpp-api-docs/deploys/686e9545b828900008767cda

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (1ad6674) to head (4c9676c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3524   +/-   ##
=======================================
  Coverage   89.96%   89.96%           
=======================================
  Files         219      219           
  Lines        7051     7051           
=======================================
  Hits         6343     6343           
  Misses        708      708           

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[owent]

LGTM, thanks.