feat(core): direct subtemplates access

[Matii96]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What is the current behavior?

Nested configs can only be accessed via parents.

What is the new behavior?

Nested configs can accessed directly.

class DbConfig {
  @IsString()
  url: string;
}

class AppConfig {
  @IsInt()
  port: number;

  @Nested(() => DbConfig)
  db: DbConfig;
}

Config.registerSync({
  template: AppConfig,
  adapter: () => ({ port: 3000, db: { url: 'db://localhost:5467' } }),
});

console.log('Db url: ' + Config.getValues(DbConfig).url) // => Db url: db://localhost:5467

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (189c944) 97.60% compared to head (ef56c1b) 97.03%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
...core/lib/manager/source-group/source-group.impl.ts	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   97.60%   97.03%   -0.57%     
==========================================
  Files          34       36       +2     
  Lines         376      405      +29     
  Branches       69       72       +3     
==========================================
+ Hits          367      393      +26     
- Misses          9       12       +3     

Flag	Coverage Δ
unittests	97.03% <98.00%> (-0.57%)	⬇️

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