feat(config): Add exporter customizers for declarative config (#6576)

[MikeGoldsmith]

Summary

Adds exporter customizers to DeclarativeConfigurationCustomizer for programmatic customization of SpanExporter, MetricExporter, and LogRecordExporter instances created from declarative configuration.

Fixes #6576

Changes

• Added addSpanExporterCustomizer(), addMetricExporterCustomizer(), addLogRecordExporterCustomizer() to DeclarativeConfigurationCustomizer
• Customizers compose in SPI registration order via mergeBiFunctionCustomizer() in DeclarativeConfigurationBuilder
• Factories apply customizers after component creation with fail-fast null checks
• Comprehensive tests: builder composition, factory application, integration end-to-end

[codecov]

Codecov Report

❌ Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.32%. Comparing base (284a64f) to head (d977871).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...or/fileconfig/DeclarativeConfigurationBuilder.java	89.65%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8081   +/-   ##
=========================================
  Coverage     90.32%   90.32%           
- Complexity     7607     7617   +10     
=========================================
  Files           839      839           
  Lines         22888    22940   +52     
  Branches       2283     2289    +6     
=========================================
+ Hits          20673    20721   +48     
- Misses         1506     1508    +2     
- Partials        709      711    +2     

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[jack-berg]

I'm supportive of these customizers because even though we want to encourage modeling all properties that matter in the opentelemetry-configuration schema, it will never be possible to cover all cases (e.g. think the executor service used to eval async requests), and we'll want an escape hatch to leverage when a a property hasn't yet been modeled in declarative config but will be.

The exporters are a good place to start because there are known gaps in the what can be expressed in declarative config (i.e. authenticators).

Let's hold off modeling additional customizers (e.g. span processors, propagators, etc) until there are concrete requests by end users or distributions which require them.

[MikeGoldsmith]

I think I've worked through all of your feedback @jack-berg, please take another look 😄