Add native HTTP/S input support (--url)

[vmvarela]

Description

Add a --url (alias -u) flag to fetch data directly from HTTP/HTTPS URLs without requiring curl.

# Fetch CSV directly from URL
sql-pipe --url https://example.com/data.csv 'SELECT * FROM t'

# Fetch JSON from API
sql-pipe --url "https://api.github.com/repos/owner/repo/issues" -I json 'SELECT * FROM t'

# With custom headers
sql-pipe --url https://api.example.com/data.json -H "Authorization: Bearer token" 'SELECT * FROM t'

Motivation

The README already shows 6+ examples using curl | sql-pipe. Baking HTTP fetching into sql-pipe eliminates the curl dependency (especially beneficial for Windows users), reduces pipe ceremony, and enables automatic format detection via Content-Type response headers.

Acceptance Criteria

• --url <url> (alias -u) flag is parsed in args.zig
• Fetch data from HTTP/HTTPS URLs using Zig's std.http.Client
• Auto-detect input format from Content-Type header (fallback to URL extension)
• Support -H / --header flag for custom HTTP headers (e.g., Authorization)
• Support --timeout flag for request timeout (default: 30s)
• Follow redirects (up to 5)
• Handle TLS certificates (system cert store on macOS/Windows, /etc/ssl/certs on Linux)
• Error messages for network failures, timeouts, non-2xx responses
• Integration tests (may need a local HTTP server fixture)
• Help text updated

Implementation Notes

• Zig 0.16 has std.http.Client — no external HTTP library needed
• TLS certificate handling is the main challenge across platforms
• Can be combined with -I for explicit format override
• Consider caching downloaded content to disk for large files (future enhancement)
• Estimated effort: 1 day