[confmap] Add featuregate to use stable expansion rules

[TylerHelmuth]

Description

This PR adds a feature gate that will handle transitioning users away from expandconverter, specifically expanding $VAR syntax.

The wholistic strategy is:

1. create a new feature gate, confmap.unifyEnvVarExpansion, that will be the single feature gate to manage unifying collector configuraiton resolution.
2. Update expandconverter to return an error if the feature gate is enabled and it is about to expand $VAR syntax.
3. Update otelcol.NewCommand to set a DefaultScheme="env" when the feature gate is enabled and no DefaultScheme is set, this handles ${VAR} syntax.
4. Separately, deprecate expandconverter.
5. Follow a normal feature gate cycle.
6. Removed the confmap.unifyEnvVarExpansion feature gates and expandconverter at the same time

Supersedes #10259

Link to tracking issue

Related to #10161
Related to #8215
Related to #7111

Testing

Unit tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.40%. Comparing base (6ca551e) to head (a31c099).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10391   +/-   ##
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  Coverage   92.39%   92.40%           
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  Files         387      387           
  Lines       18310    18323   +13     
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+ Hits        16918    16931   +13     
  Misses       1046     1046           
  Partials      346      346           

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[mx-psi]

One small change and it's good to go from my side

[codeboten]

Needs rebasing