Add support for "balance" option

Closes #462

src/main/java/io/tarantool/driver/api/space/options/SelectOptions.java

@@ -1,5 +1,6 @@
 package io.tarantool.driver.api.space.options;
 
+import io.tarantool.driver.api.space.options.crud.OperationWithBalanceOptions;
 import io.tarantool.driver.api.space.options.crud.OperationWithBatchSizeOptions;
 import io.tarantool.driver.api.space.options.crud.OperationWithBucketIdOptions;
 import io.tarantool.driver.api.space.options.crud.OperationWithFieldsOptions;
@@ -21,5 +22,6 @@
 public interface SelectOptions<T extends SelectOptions<T>>
     extends OperationWithBucketIdOptions<T>, OperationWithTimeoutOptions<T>, OperationWithFieldsOptions<T>,
             OperationWithModeOptions<T>, OperationWithBatchSizeOptions<T>, OperationWithYieldEveryOptions<T>,
-            OperationWithForceMapCallOptions<T>, OperationWithFullScanOptions<T>, OperationWithPreferReplicaOptions<T> {
+            OperationWithForceMapCallOptions<T>, OperationWithFullScanOptions<T>, OperationWithPreferReplicaOptions<T>,
+            OperationWithBalanceOptions<T> {
 }

src/main/java/io/tarantool/driver/api/space/options/crud/OperationWithBalanceOptions.java

@@ -0,0 +1,33 @@
+package io.tarantool.driver.api.space.options.crud;
+
+import java.util.Optional;
+
+import io.tarantool.driver.api.space.options.Options;
+import io.tarantool.driver.api.space.options.Self;
+import io.tarantool.driver.api.space.options.crud.enums.ProxyOption;
+
+/**
+ * Base interface for all operation options that may have a configurable "balance" option.
+ *
+ * @author Belonogov Nikolay
+ */
+public interface OperationWithBalanceOptions<T extends OperationWithBalanceOptions<T>> extends Options, Self<T> {
+
+    /**
+     * Sets "balance" option value to true. If true then use replica according to vshard load balancing policy. Default
+     * value is false.
+     *
+     * @return this options instance.
+     */
+    default T balance() {
+        addOption(ProxyOption.BALANCE, true);
+        return self();
+    }
+
+    /**
+     * @return "balance" option value.
+     */
+    default Optional<Boolean> getBalance() {
+        return getOption(ProxyOption.BALANCE, Boolean.class);
+    }
+}

src/main/java/io/tarantool/driver/api/space/options/crud/enums/ProxyOption.java

@@ -31,7 +31,9 @@ public enum ProxyOption {
 
     FULL_SCAN("fullscan"),
 
-    PREFER_REPLICA("prefer_replica");
+    PREFER_REPLICA("prefer_replica"),
+
+    BALANCE("balance");
 
     private final String name;
 

src/test/java/io/tarantool/driver/integration/proxy/options/ProxySpaceSelectOptionsIT.java

@@ -302,4 +302,20 @@ public void withPreferReplicaTest() {
 
         assertEquals(true, ((HashMap<?, ?>) crudSelectOpts.get(0)).get(ProxyOption.PREFER_REPLICA.toString()));
     }
+
+    @Test
+    public void withBalanceTest() {
+        TarantoolSpaceOperations<TarantoolTuple, TarantoolResult<TarantoolTuple>> operations =
+            client.space(TEST_SPACE_NAME);
+
+        SelectOptions<ProxySelectOptions> options = ProxySelectOptions.create().balance();
+
+        assertTrue(options.getBalance().isPresent());
+        assertTrue(options.getBalance().get());
+
+        operations.select(Conditions.any(), options).join();
+        List<?> crudSelectOpts = client.eval("return crud_select_opts").join();
+
+        assertEquals(true, ((HashMap<?, ?>) crudSelectOpts.get(0)).get(ProxyOption.BALANCE.toString()));
+    }
 }