Add HTTP header support for authenticated downloads

[Copilot]

Adds ability to specify custom HTTP headers via CLI flag or environment variable, enabling authenticated downloads and other header-based use cases.

Changes

• CLI flag: Added -H/--header flag accepting "Key: Value" format (repeatable)
• Environment variable: PGET_HEADERS as JSON map (e.g., {"Authorization":"Bearer token"})
• Config: New HeadersToMap() function parses CLI headers and merges with env vars
• Client: Modified PGetHTTPClient.Do() to apply custom headers while preserving User-Agent
• Tests: Added validation for header parsing and transmission

Usage

# CLI flags
pget -H "Authorization: Bearer token123" https://api.example.com/file.tar ./dest

# Environment variable
PGET_HEADERS='{"Authorization":"Bearer token123"}' pget https://api.example.com/file.tar ./dest

# Combine both (CLI takes precedence for duplicate keys)
PGET_HEADERS='{"X-Env":"value"}' pget -H "X-CLI: value" https://example.com/file.tar ./dest

Headers are applied to all HTTP requests. User-Agent is always set to pget version and cannot be overridden.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow providing http header</issue_title>
<issue_description>It would be nice to have an option to specify http headers. Maybe limit to http headers that won't conflict with pget.

For example: This would enable authentication with Authorization header</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Allow providing http header #246

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