Add support for OAuth2 authentication

[virtualzx-nad]

Original issue: httpie#1538

Problem:
HTTPie currently lacks built-in support for OAuth2 authentication flows.

Requirements:

1. Implement OAuth2 authentication support
2. Support different grant types (authorization code, client credentials, etc.)
3. Handle token refresh
4. Add secure token storage
5. Include tests
6. Add documentation with examples

Difficulty: Moderate
Type: Feature