feat: Convenience for custom headers for orchestration and OpenAI

[Jonas-Isr]

Context

AI/ai-sdk-java-backlog#300

This PR introduces withHeader methods for OrchestrationClient and OpenAiClient. These methods either take tow strings (key and value of the header) or a Header object and return a new client where this header will be added to each call made from this client. This allows for convenient ways of adding custom headers to single calls (or groups thereof).

var client = new OrchestrationClient();

var result = client.withHeader("foo", "bar").chatCompletion(prompt, config);

Feature scope:

• added withHeader methods to OrchestrationClient
• added withHeader methods to OpenAiClient

Definition of Done

• Functionality scope stated & covered
• Tests cover the scope above
• Error handling created / updated & covered by the tests above
• Aligned changes with the JavaScript SDK
• Documentation updated
  • docs PR
• Release notes updated