Add CloudKit Server-to-Server API Support

[meszmate]

Summary

• Add full CloudKit server-to-server API client with support for records, zones, subscriptions, assets, users, and APNs tokens
• Fix bug in url.go:56 where responseMode was assigned instead of responseType
• Extract shared parseECPrivateKey() function for reuse between Apple Sign-In and CloudKit
• Standardize HTTP client timeout to 10s in both New() and NewB64()
• Replace deprecated io/ioutil usage with io in tests
• Fix lint warnings (staticcheck QF1008) in mock implementations
• Add comprehensive test suite (47 tests) with mocked HTTP clients
• Add GitHub Actions CI with Go 1.24/1.25 matrix, race detector, coverage, and golangci-lint
• Rewrite README with corrected examples (IDToken/Subject) and full CloudKit documentation
• Add proper .gitignore for Go projects