feat: add X-Glean headers for experimental features and deprecation testing

[travis-hoover-glean]

Summary

• Add XGleanHeadersHook beforeRequest hook that sets X-Glean-Exclude-Deprecated-After and X-Glean-Experimental headers
• Introduce new SDK options excludeDeprecatedAfter and includeExperimental with corresponding environment variable support
• Add documentation for testing experimental features and deprecation testing

This is the Java SDK equivalent of gleanwork/api-client-typescript#95

Test plan

• Verify headers are set correctly when using SDK constructor options
• Verify headers are set correctly when using environment variables
• Confirm environment variables take precedence over SDK options
• Ensure no headers are set when neither option is configured

// Test with SDK options
Glean glean = Glean.builder()
        .apiToken(System.getenv("GLEAN_API_TOKEN"))
        .instance(System.getenv("GLEAN_INSTANCE"))
        .excludeDeprecatedAfter("2026-10-15")
        .includeExperimental(true)
        .build();

// Or using environment variables
// X_GLEAN_EXCLUDE_DEPRECATED_AFTER=2027-10-15
// X_GLEAN_INCLUDE_EXPERIMENTAL=true