Introduce lookaside_http_version config option to control HTTP protocol
version used for source file downloads and uploads. Valid values:
- 'auto' (default): Let curl negotiate (may use HTTP/2)
- '1.1': Force HTTP/1.1 (better for high-latency connections)
- '2': Force HTTP/2 (better for low-latency, multiplexing)

Also include UPLOAD_BUFFERSIZE optimization (1MB buffer) with hasattr
check for compatibility with older libcurl versions (RHEL 7).

This allows users to optimize transfer performance based on their
network characteristics while maintaining backward compatibility.

Configuration example:
[fedpkg]
lookaside_http_version = 1.1

Inspired-by: https://pagure.io/rpkg/pull-request/772
Assisted-by: Claude Sonnet 4.5 noreply@anthropic.com