Validate configuration source keys and overrides

internal/config/validate.go

@@ -53,6 +53,9 @@ func Validate(cfg Config) error {
 		if strings.TrimSpace(source) == "" {
 			return fmt.Errorf("missing_source.sources contains an empty source name")
 		}
+		if !IsSupportedMissingSource(source) {
+			return fmt.Errorf("missing_source.sources.%s is not a supported optional source", source)
+		}
 		if err := validatePolicy("missing_source.sources."+source, policy); err != nil {
 			return err
 		}