Add support for loading headers from files from Issue #1595

[anaryer]

Description: This PR adds support for reading headers from a file using the @ syntax. It handles multiple headers per file and empty lines, and raises a ParseError for malformed headers. Updated RequestItems.from_args to handle multi-header files. Added test_headers.py to verify parsing and CLI usage. Testing: Ran test_headers.py to confirm parsing and error handling. Verified with CLI using http get https://httpbin.org/headers @headers.txt