feat: add HTTP/HTTPS authentication and SSL validation support

[JusterZhu]

Summary

Adds HTTP/HTTPS compatibility features (SSL validation, proxy, timeouts, retry) and multiple authentication methods (Bearer, Basic, ApiKey, HMAC) to GeneralUpdate.Maui.Android.

Changes

New files (6)

• IHttpAuthProvider + ISslValidationPolicy interfaces
• AuthScheme enum (Hmac/Bearer/ApiKey/Basic)
• 5 auth providers (NoOp, BearerToken, ApiKey, Basic, Hmac) + HttpAuthProviderFactory
• StrictSslValidationPolicy / AllowAllSslValidationPolicy
• HttpDownloadOptions configuration record
• publish-nuget.yml CI workflow (mirrors GeneralUpdate.Avalonia pattern)

Modified files (4)

• UpdatePackageInfo.cs — 5 nullable auth fields for per-package authentication (overrides global)
• HttpRangeDownloader.cs — Auth injection into HTTP GET requests, IDisposable
• GeneralUpdateBootstrap.cs — Optional HttpDownloadOptions parameter
• AndroidBootstrap.cs — Disposes downloader if IDisposable

CI workflow

• SemVer 2.0 validation
• Android workload installation
• Build, test, pack
• Git tag + GitHub Release
• NuGet.org Trusted Publishing (OIDC)

Design

• Fully backward-compatible (all new params optional, default behavior identical)
• Auth priority: per-package > global > none
• Instance-based (no static global state)
• No dependency on GeneralUpdate.Core

Verification

• ✅ Library builds: 0 warnings, 0 errors
• ✅ All 8 existing unit tests pass
• ✅ Test project builds: 0 warnings, 0 errors

Closes #14 (feature request for auth support)