feat: Add LoadFromConfiguration for appsettings.json support

[JusterZhu]

Summary

The refactoring plan specifies a LoadFromConfiguration(IConfiguration config) method on GeneralUpdateBootstrap for loading settings from appsettings.json, environment variables, and command-line args. This is not yet implemented.

Expected API

// Load from IConfiguration (Microsoft.Extensions.Configuration)
new GeneralUpdateBootstrap()
    .LoadFromConfiguration(builder.Configuration)
    .LaunchAsync();

// Load from appsettings.json section
new GeneralUpdateBootstrap()
    .LoadFromConfiguration(config.GetSection("GeneralUpdate"))
    .LaunchAsync();

Expected Behavior

1. Map IConfiguration keys to UpdateOptions and Configinfo fields
2. Priority: code .Option() calls override configuration values
3. Support standard appsettings.json bindings:
   • GeneralUpdate:AppType
   • GeneralUpdate:UpdateUrl
   • GeneralUpdate:AppSecretKey
   • GeneralUpdate:DiffMode
   • GeneralUpdate:Silent, etc.
4. Chainable: .LoadFromConfiguration(config).Option(UpdateOptions.Silent, true) — code overrides config

Benefits

• Zero-code configuration for common scenarios
• Standard .NET configuration pattern
• Enables environment-specific configs (appsettings.Development.json, appsettings.Production.json)

Ref

Refactoring plan v2: Section 2.3, "LoadFromConfiguration"