Refactor authentication handlers and middleware

internal/auth/credentials.go

@@ -0,0 +1,11 @@
+package auth
+
+import "errors"
+
+func (a *Authenticator) verifyCredentials(login, password string) error {
+
+	if login == "admin" && password == "secret" {
+		return nil
+	}
+	return errors.New("wrong credentials")
+}