Add Linux PlatformProxyController support

[umeow0716]

Summary

• Add PlatformProxyController, ProxySettings, ProxyRule, and ProxySchemeFilter to webview_platform_interface.
• Add the public ProxyController wrapper from webview_all.
• Implement Linux proxy override support in webview_all_linux using WebKitGTK network proxy settings.
• Add export and platform tests for the new proxy controller surface.

Validation

• Verified the branch compares cleanly against upstream main with 2 commits ahead and no commits behind.
• Manually validated in a downstream Linux Flutter desktop app using the forked webview_all packages.