IDEV-2272 and IDEV-2273: support header auth for iris endpoints and change default Feeds auth behavior

[jbabac]

Changes:

1. Change default Feeds auth behavior
   a. always_sign_api_key is False by default unless explicitly set by user
   b. header_authentication is True by default unless explicitly set by user
   c. Since RTTF endpoints don’t support signed API keys with any auth method, if the user sets always_sign_api_key=True for RTTF, endpoints throw an error that says “Real Time Threat Feeds do not support signed API keys.”
   d. For non-RTTF endpoints, always_sign_api_key is set True unless explicitly set by user

2. Support X-Api-Key Header Auth for Iris Endpoints

Unit Tests:

E2E Tests:

[wesleya]

Nice job JD!