feat: complete HTTP authentication system with 5 built-in providers

[JusterZhu]

Problem

The authentication system was incomplete:

1. No Basic Auth support (common in enterprise environments)
2. No explicit way to select auth method — relied on implicit string scheme matching
3. Factory priority bug: HMAC always won because AppSecretKey is required
4. HMAC took precedence over explicit user choices

Solution (PR #495)

New features

• BasicAuthProvider — RFC 7617 Basic Authentication with EncodeCredential helper
• AuthScheme enum — explicit type-safe selection: Hmac | Bearer | ApiKey | Basic
• BasicUsername / BasicPassword — new config fields flowing through full pipeline
• Rename UpdateAuth to HttpAuth for naming consistency

Bug fix

• Factory priority fixed: explicit AuthScheme now takes precedence over HMAC default

Files changed

10 files, +285/-24 lines in GeneralUpdate.Core