[TT-16429]: Add custom trace header configuration for Gateway

[imogenkraak]

Description

This PR adds support for custom trace headers (e.g., X-Correlation-ID, X-Request-ID) to enable organisations with legacy correlation ID systems to integrate with OpenTelemetry without breaking existing trace continuity.

Related Issue

TT-26429

Motivation and Context

Test Coverage For This Change

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)
• Documentation updates or improvements.

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[imogenkraak]

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