docs(java): add Javadoc to OpenTelemetryConfiguration

[S-anskriti]

Summary

Adds missing class-level and method-level Javadoc to
OpenTelemetryConfiguration.java.

Motivation

The public API of OpenTelemetryConfiguration had no Javadoc on the
class, constructor, or getter methods, making it harder for new
contributors to understand parameter expectations, return value
semantics (especially the empty Optional cases), and the relationship
between plugin settings and OpenTelemetry SDK properties.

Changes

• Added class-level Javadoc explaining the purpose of the class
• Added constructor Javadoc with full @param for all 7 parameters
  including the validation rule on the endpoint format
• Added method Javadoc for:
  • getEndpoint() — OTLP URL format and empty Optional semantics
  • getServiceName() — relationship to service.name resource attribute
  • getServiceNamespace() — relationship to service.namespace resource attribute
  • getAuthentication() — supported authentication mechanisms
  • getTrustedCertificatesPem() — when and how to use custom TLS certs
  • getDisabledResourceProviders() — maps to otel.java.disabled.resource.providers

Testing

Verified mvn compile succeeds with no new warnings.
No functional changes.

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Note: I am a GSoC 2026 applicant interested in the
Use OpenTelemetry for Jenkins Jobs on ci.jenkins.io
project.

[S-anskriti]

Hi @shivaylamba and @krisstern . This is my second contribution to the plugin as part of my GSoC 2026 application for the OpenTelemetry project. Would love any feedback!