[TT-13680] feat: support reading headers from Kubernetes Secrets

[buraksekili]

Add support for reading opentelemetry header values from k8s secrets while maintaining backward compatibility with direct header values.
This enables secure management of sensitive header values like authentication tokens.

Changes:

• Headers can now be specified either as direct string values or references to Secret values
• Support mixing both direct values and Secret references in the same headers map
• Updated values.yaml documentation with examples of both approaches
• Headers continue to be formatted as "key1:value1,key2:value2" in final output

Example:

headers:
  X-Custom-Header: "direct-value"
  Authorization:
    fromSecret:
      name: "auth-secret"
      key: "auth-token"

Description

Related Issue

Resolves #363
https://tyktech.atlassian.net/browse/TT-13680

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.

Checklist

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