[mdatagen] Add Converter and Provider support

[evan-bradley]

Description

Adds support for confmap Converters and Providers to mdatagen, which allows us to declare stability levels for these module types.

Helps unblock open-telemetry/opentelemetry-collector-releases#546.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.18%. Comparing base (2b5fa0e) to head (1e9117c).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12467      +/-   ##
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- Coverage   92.19%   92.18%   -0.01%     
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  Files         465      465              
  Lines       25194    25171      -23     
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- Hits        23228    23205      -23     
  Misses       1570     1570              
  Partials      396      396              

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[bogdandrutu]

What is special for them vs the generic "pkg"?

[evan-bradley]

As a class of module: currently only the prefix (pkg/x vs. converter/x or provider/x) which are used in the labels in the generated readme. As a target for a certain level of stability: They're like extensions, so stability doesn't apply to a particular signal.

Considering the bulk of this change is just adding two new items to a few slices, I'm more inclined to explicitly add these as opposed to come up with exceptions for existing classes or make them work for more general cases. None of the existing options are really a great fit.