feat(go): abstract logging behind an interface

[nicklasl]

Summary

• accept a small Logger interface instead of requiring *slog.Logger
• thread custom logging through current resolver and multi-destination logging paths
• use a no-op logger when none is configured
• document the interface and keep test loggers race-safe

Attribution

This revamps and supersedes #494 by @lbcjbb. The original feature commit retains Jean-Baptiste Bronisz as its Git author.

Testing

• go test ./confidence/... -skip '^(TestFlagResolve_WithMaterialized|TestFlagResolve_WithEncryptedState|TestFlagLogs_)'
• go vet ./...
• focused -race test for asynchronous provider shutdown logging

The live e2e tests require Confidence credentials and are left to CI.