FORK: FDB 649 improve support for optional + default

[mcakircali]

Description

Alternative implementation of "optional-default" work

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/fdb/pull-requests/PR-255

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.32%. Comparing base (95f9677) to head (465cd07).

Files with missing lines	Patch %	Lines
src/fdb5/rules/Matcher.cc	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                      @@
##           feature/optional-default     #255      +/-   ##
============================================================
+ Coverage                     71.23%   71.32%   +0.09%     
============================================================
  Files                           376      376              
  Lines                         23789    23791       +2     
  Branches                       2488     2483       -5     
============================================================
+ Hits                          16946    16970      +24     
+ Misses                         6843     6821      -22     

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