Support custom HTTP headers for all API requests

[ttech8485-star]

Some APIs require additional HTTP headers for authentication, versioning, localization, or custom integrations. Currently, there is no straightforward way to define default headers that are automatically included with every request.

1. Allow configuring default HTTP headers globally.
2. Support overriding or extending headers on individual requests.
3. Preserve existing default headers unless explicitly replaced.
4. Validate header names and values to prevent invalid configurations.

This would make it easier to integrate with a wider range of APIs, reduce repetitive configuration, and improve flexibility for applications with custom authentication or request requirements.