feat: add default_headers support to config

[Marias03]

Closes #1660

What does this PR do?

Adds support for default_headers in HTTPie's config file.
Users can now set headers that will be automatically included in every request.

How to use it?

Add to your config.json:

{
    "default_headers": {
        "Content-Type": "application/json"
    }
}

These headers can still be overridden per-request by the user.

Changes

• httpie/config.py: Added default_headers to Config.DEFAULTS and a new property
• httpie/client.py: Applied default_headers from config in make_request_kwargs
• tests/test_config.py: Added two tests for the new feature