Move declarative config experimental types to internal package

[jack-berg]

Part of #8402.

Adjusts jsonschema2pojo generation logic to move experimental types (i.e. those with Experimental* prefix) to the *.model.internal* package.

This is a prereq to us stabilizing the APIs because experimental types are subject to breaking changes that violate our versioning policy.

Still TODO in a followup: some stable types reference experimental types. For example:

opentelemetry-java/sdk-extensions/declarative-config/src/main/java/io/opentelemetry/sdk/autoconfigure/declarativeconfig/model/ResourceModel.java

Lines 70 to 74 in 77eb68d

	@JsonProperty("detection/development")
	@Nullable
	public ExperimentalResourceDetectionModel getDetectionDevelopment() {
	return detectionDevelopment;
	}

We need to figure out a way to hide these accessors from the API, since they too are subject to breaking changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.91%. Comparing base (77eb68d) to head (25b2319).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8530      +/-   ##
============================================
- Coverage     90.94%   90.91%   -0.03%     
+ Complexity    10210    10207       -3     
============================================
  Files          1013     1013              
  Lines         27175    27175              
  Branches       3184     3184              
============================================
- Hits          24714    24706       -8     
- Misses         1734     1742       +8     
  Partials        727      727              

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