[configoptional] Mark 'configoptional.AddEnabledField' as beta

[mx-psi]

Description

Marks configoptional.AddEnabledField as beta. I have verified that it would work correctly for cookies configuration and made a PoC for keepalives (I will push a PR for it after this PR has been merged).

I spent some time updating open-telemetry/opentelemetry-specification/issues/4344 and filed open-telemetry/opentelemetry-specification/pull/4723 as a way to explicitly state that the guidance only applies to environment variables. The issue remains unresolved, but given the current usages of enabled (see open-telemetry/opentelemetry-specification#4344 (comment)) I don't see a reason to not go forward with this.

Link to tracking issue

Updates #14021

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.22%. Comparing base (a6a1766) to head (e6e1a8f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14136      +/-   ##
==========================================
- Coverage   92.25%   92.22%   -0.03%     
==========================================
  Files         657      657              
  Lines       41068    41068              
==========================================
- Hits        37888    37876      -12     
- Misses       2176     2184       +8     
- Partials     1004     1008       +4     

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