[configoptional] Add a confmap.Marshaler implementation

[mx-psi]

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Description

Adds a confmap.Marshaler implementation for configoptional.Optional

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 46.42857% with 15 lines in your changes missing coverage. Please review.

Project coverage is 91.49%. Comparing base (e7b140f) to head (2b0a575). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
confmap/confmap.go	25.00%	8 Missing and 1 partial :warning:
config/configoptional/optional.go	50.00%	4 Missing and 2 partials :warning:

:x: Your patch status has failed because the patch coverage (46.42%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13196      +/-   ##
==========================================
- Coverage   91.54%   91.49%   -0.06%     
==========================================
  Files         522      522              
  Lines       28955    28980      +25     
==========================================
+ Hits        26508    26514       +6     
- Misses       1931     1946      +15     
- Partials      516      520       +4     

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[mx-psi]

Since we discussed this on the Collector stability meeting and the new test was not deemed strictly necessary to merge this PR I am going to merge it without the test to unblock further work in configoptional and friends and work on it later