Replace jsonschema2pojo with hand rolled version

[jack-berg]

I've recently made a handful of changes to heavily customize the behavior of jsonschema2pojo. Along the way, I started increasingly feeling like I was working against the currents - jumping through hoops to get the output I wanted. And I've been wondering what it would look like to rip out jsonschema2pojo in favor of hand rolling.

This PR does just that.

Its faithfully recreates the generated output of jsonschema2pojo with just a few exceptions:

• Change @Generated("jsonschema2pojo") to @Generated("io.opentelemetry.gradle.DeclarativeConfigPojoGenerator")
• Change enums to top level instead of nested. jsonschema2pojo has been doing something weird with enum types. It adds them as inner classes to the class that first references them. Given the strangeness, uselessness, and that recreating added complexity, I opted to fix in the handrolled version.
• Enum types now include *Model suffix

[codecov]

Codecov Report

❌ Patch coverage is 78.10945% with 44 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.62%. Comparing base (6f6e146) to head (ff8596d).

Files with missing lines	Patch %	Lines
...re/declarativeconfig/model/AttributeTypeModel.java	86.36%	2 Missing and 1 partial ⚠️
...e/declarativeconfig/model/ExemplarFilterModel.java	82.35%	2 Missing and 1 partial ⚠️
...odel/ExporterDefaultHistogramAggregationModel.java	81.25%	2 Missing and 1 partial ⚠️
...nfig/model/ExporterTemporalityPreferenceModel.java	82.35%	2 Missing and 1 partial ⚠️
...e/declarativeconfig/model/InstrumentTypeModel.java	85.71%	2 Missing and 1 partial ⚠️
...declarativeconfig/model/OtlpHttpEncodingModel.java	81.25%	2 Missing and 1 partial ⚠️
...e/declarativeconfig/model/SeverityNumberModel.java	92.10%	2 Missing and 1 partial ⚠️
...xperimentalPrometheusTranslationStrategyModel.java	83.33%	2 Missing and 1 partial ⚠️
...declarativeconfig/model/AlwaysOffSamplerModel.java	0.00%	1 Missing ⚠️
.../declarativeconfig/model/AlwaysOnSamplerModel.java	0.00%	1 Missing ⚠️
... and 18 more

❌ Your patch check has failed because the patch coverage (78.10%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8600      +/-   ##
============================================
- Coverage     91.62%   91.62%   -0.01%     
- Complexity    10321    10361      +40     
============================================
  Files          1013     1021       +8     
  Lines         27287    27285       -2     
  Branches       3203     3203              
============================================
- Hits          25001    24999       -2     
  Misses         1559     1559              
  Partials        727      727              

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