feat: add support for Gentoo Linux in HTTP client and package management

[coleleavitt]

This pull request adds support for Gentoo Linux as a recognized platform in the codebase. It introduces Gentoo-specific configuration, package lists, and package manager handling, ensuring that Gentoo can be properly detected and managed alongside other Linux distributions.

Gentoo Linux support:

• Added gentoo to the list of known Linux platforms in PlatformDefinition and updated platform detection logic to recognize Gentoo systems. [1] [2]
• Included Gentoo in the list of platforms handled by the HTTP client, enabling API compatibility.

Package management and configuration:

• Added Gentoo-specific package list and mapped the package manager to emerge in the Linux platform implementation. [1] [2] [3]
• Implemented package installation checks using Portage's qlist for Gentoo.