[confmap] Automatically invoke Validate() during Conf.Unmarshal

[TylerHelmuth]

Description

Duplicate component.ConfigValidator into confmap. Add capability for ConfigValidator.Validate to be invoked as part of unmarshal. Add a new UnmarshalOption to allow controlling if validation is invoked.

If we like this feature, we'll need to do some refactoring to how otelcol unmarshals if we want to take advantage of it. See #11524 (comment). After implementing it I'm still not convinced this solution is better than what we already have.

Link to tracking issue

Related to #11524

Testing

Added new unit tests. Tested manually using local build.

Documentation

Added godoc comments.

[codecov]

Codecov Report

Attention: Patch coverage is 88.70968% with 7 lines in your changes missing coverage. Please review.

Project coverage is 91.66%. Comparing base (b682d8e) to head (83c701c).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
confmap/confmap.go	88.70%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12031      +/-   ##
==========================================
- Coverage   91.68%   91.66%   -0.02%     
==========================================
  Files         455      455              
  Lines       24053    24099      +46     
==========================================
+ Hits        22052    22091      +39     
- Misses       1629     1634       +5     
- Partials      372      374       +2     

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[bogdandrutu]

There are some problems with this design, because the "validation" chain may be broken by custom "Unmarshaler" implementations which can lead to invalid configs being used:

struct A {
  struct B {
    struct C {}
    field D
  }
}

func (B) Unmarshal(...) error {
   // Here I may endup manually unmarshaling D and not calling Validate on C/D.
}  

Other problems can be if config is initially unmarshalled in a map then moved to the struct, etc.

To prove this works, we need lots of tests for lots of different usage patterns.

[TylerHelmuth]

@bogdandrutu I was able to test the custom unmarshal scenario with the otlp receiver and confirmed it works since the custom unmarshal implementation invokes conf.Unmarshal.

If the custom unmarshaller does not invoked conf.Unmarshal it will still work since validate is ultimately called on the top level object that is being unmarshalled into.

I've added a tests to demonstrate these 2 scenarios.