multi: add CreditAccount & DebitAccount endpoints

This commit introduces the structure for new endpoints in the accounts
subsystem, enabling balance adjustments by crediting or debiting a
specified amount. This contrasts with the existing `UpdateAccount`
implementation, which directly sets the balance to a fixed value.

For more details on the drawbacks of the current implementation, see
[issue #648](#648).

The actual implementation of the endpoints will be introduced in
subsequent commits.

Author: ViktorT-11

accounts/rpcserver.go

@@ -130,6 +130,28 @@ func (s *RPCServer) UpdateAccount(ctx context.Context,
 	return marshalAccount(account), nil
 }
 
+// CreditAccount increases the balance of an existing account in the account
+// database, by the given amount.
+func (s *RPCServer) CreditAccount(ctx context.Context,
+	req *litrpc.UpdateAccountBalanceRequest) (*litrpc.Account, error) {
+
+	log.Infof("[creditaccount] id=%s, label=%v, amount=%d",
+		req.Id, req.Label, req.Amount)
+
+	return nil, fmt.Errorf("not implemented")
+}
+
+// DebitAccount decreases the balance of an existing account in the account
+// database, by the given amount.
+func (s *RPCServer) DebitAccount(ctx context.Context,
+	req *litrpc.UpdateAccountBalanceRequest) (*litrpc.Account, error) {
+
+	log.Infof("[debitaccount] id=%s, label=%v, amount=%d",
+		req.Id, req.Label, req.Amount)
+
+	return nil, fmt.Errorf("not implemented")
+}
+
 // ListAccounts returns all accounts that are currently stored in the account
 // database.
 func (s *RPCServer) ListAccounts(ctx context.Context,