[cmd/mdatagen] Handle default values for configuration fields in generated code.

[jkoronaAtCisco]

Description

This PR extends mdatagen's config code generator to automatically produce createDefaultConfig() (and per-definition createDefaultXxx()) factory functions from default: values declared in the JSON Schema config.

Supported default types:

• string → quoted Go string literal
• string + format: duration → time package expression (e.g. 10time.Second, 1time.Hour + 30*time.Minute)
• integer / number → numeric literal
• boolean → true / false
• x-optional: true fields → wrapped with configoptional.Some(...)
• x-pointer: true fields → local variable + address-of
• Nested struct definitions (inline objects, $ref, array-of-objects) → delegates to a createDefaultXxx() per definition

Currently not supported types: object, array (non-object items), date-time strings, x-customType fields, external $refs. Support to those types will be added in separate PRs.

Link to tracking issue

#14560

Testing

1. Unit tests coverage
2. Sample components updated

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.17%. Comparing base (6a56478) to head (2f587fb).

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/cfggen/generation.go	85.20%	21 Missing and 4 partials ⚠️
...datagen/internal/samplescraper/generated_config.go	0.00%	7 Missing ⚠️
...atagen/internal/samplereceiver/generated_config.go	0.00%	4 Missing ⚠️

❌ Your patch check has failed because the patch coverage (80.00%) 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   #15018      +/-   ##
==========================================
- Coverage   91.22%   91.17%   -0.05%     
==========================================
  Files         697      697              
  Lines       44627    44807     +180     
==========================================
+ Hits        40710    40853     +143     
- Misses       2773     2806      +33     
- Partials     1144     1148       +4     

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