feat: zero-config SetSource() API with manifest.json auto-discovery

[JusterZhu]

Problem

Currently developers must hand-write a 19-field Configinfo object to configure GeneralUpdateBootstrap. Identity metadata (app name, version, update path) is mixed with secrets (URL, key), making configuration error-prone for new users.

Solution

Add SetSource(url, key, token?, scheme?, reportUrl?) as a minimal entry-point. Secrets are supplied in code. Identity metadata is auto-discovered from generalupdate.manifest.json (generated by GeneralUpdate.Tools).

Implementation

See PR: #463