(feat-351) add oltp headers

[hangweizhang]

feat(studio): add OTLP headers support for authentication

Add support for configuring OTLP headers in agentscope-extensions-studio
to enable authentication with external tracing systems like Langfuse.

Changes:

• Add otlpHeaders field to StudioConfig with getter method
• Add addOtlpHeader() and otlpHeaders() builder methods to StudioConfig
• Add corresponding methods to StudioManager.Builder for convenience
• Pass configured headers to TelemetryTracer during initialization
• Add comprehensive test cases for header configuration

This closes #351 by allowing users to configure authentication headers
when reporting trace data via OTLP to systems requiring authentication.

Example usage for Langfuse:

StudioManager.init()
    .studioUrl("https://cloud.langfuse.com")
    .tracingUrl("https://cloud.langfuse.com/api/public/otel/v1/traces")
    .addOtlpHeader("Authorization", "Basic " + base64Credentials)
    .initialize()
    .block();

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../java/io/agentscope/core/studio/StudioManager.java	50.00%	4 Missing ⚠️

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[hangweizhang]

@LearningGp thx for review this pr for support oltp headers #351