Support Spark SHOW TABLES command (unitycatalog#293)

**Description of changes**

<!-- Please state what you've changed and how it might affect the users.
-->
Support Spark SHOW TABLES command

connectors/spark/src/main/scala/io/unitycatalog/connectors/spark/UCSingleCatalog.scala

@@ -2,7 +2,7 @@ package io.unitycatalog.connectors.spark
 
 import io.unitycatalog.client.{ApiClient, ApiException}
 import io.unitycatalog.client.api.{TablesApi, TemporaryTableCredentialsApi}
-import io.unitycatalog.client.model.{AwsCredentials, GenerateTemporaryTableCredential, TableOperation, TableType}
+import io.unitycatalog.client.model.{AwsCredentials, GenerateTemporaryTableCredential, ListTablesResponse, TableOperation, TableType}
 
 import java.net.URI
 import java.util
@@ -14,6 +14,7 @@ import org.apache.spark.sql.connector.expressions.Transform
 import org.apache.spark.sql.types.{DataType, StructField, StructType}
 import org.apache.spark.sql.util.CaseInsensitiveStringMap
 
+import scala.collection.convert.ImplicitConversions._
 import scala.jdk.CollectionConverters._
 
 /**
@@ -33,7 +34,7 @@ class UCSingleCatalog extends TableCatalog {
 
   override def name(): String = deltaCatalog.name()
 
-  override def listTables(namespace: Array[String]): Array[Identifier] = ???
+  override def listTables(namespace: Array[String]): Array[Identifier] = deltaCatalog.listTables(namespace)
 
   override def loadTable(ident: Identifier): Table = deltaCatalog.loadTable(ident)
 
@@ -91,7 +92,18 @@ private class UCProxy extends TableCatalog {
     this.name
   }
 
-  override def listTables(namespace: Array[String]): Array[Identifier] = ???
+  override def listTables(namespace: Array[String]): Array[Identifier] = {
+    if (namespace.length > 1) {
+      throw new ApiException("Nested namespaces are not supported:  " + namespace.mkString("."))
+    }
+    val catalogName = this.name
+    val schemaName = namespace.head
+    val maxResults = 0
+    val pageToken = null
+    val response: ListTablesResponse = tablesApi.listTables(catalogName, schemaName, maxResults, pageToken)
+    response.getTables.toSeq.map(table => Identifier.of(namespace, table.getName)).toArray
+  }
+
 
   override def loadTable(ident: Identifier): Table = {
     val t = try {

connectors/spark/src/test/java/io/unitycatalog/connectors/spark/SparkIntegrationTest.java

@@ -1,8 +1,7 @@
 package io.unitycatalog.connectors.spark;
 
 import static io.unitycatalog.server.utils.TestUtils.*;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.*;
 
 import io.unitycatalog.client.ApiException;
 import io.unitycatalog.client.model.*;
@@ -19,6 +18,7 @@
 import java.io.IOException;
 import java.util.*;
 import org.apache.spark.network.util.JavaUtils;
+import org.apache.spark.sql.AnalysisException;
 import org.apache.spark.sql.Row;
 import org.apache.spark.sql.SparkSession;
 import org.junit.jupiter.api.BeforeEach;
@@ -143,6 +143,24 @@ public void testCredentialDelta() throws ApiException, IOException {
     session.stop();
   }
 
+  @Test
+  public void testShowTables() throws ApiException, IOException {
+    createCommonResources();
+    SparkSession session = createSparkSessionWithCatalogs(SPARK_CATALOG);
+    setupExternalParquetTable(PARQUET_TABLE, new ArrayList<>(0));
+
+    Row[] tables = (Row[]) session.sql("SHOW TABLES in " + SCHEMA_NAME).collect();
+    assertEquals(tables.length, 1);
+    assertEquals(tables[0].getString(0), SCHEMA_NAME);
+    assertEquals(tables[0].getString(1), PARQUET_TABLE);
+
+    AnalysisException exception =
+        assertThrows(AnalysisException.class, () -> session.sql("SHOW TABLES in a.b.c").collect());
+    assertTrue(exception.getMessage().contains("a.b.c"));
+
+    session.stop();
+  }
+
   private String generateTableLocation(String catalogName, String tableName) throws IOException {
     return new File(new File(dataDir, catalogName), tableName).getCanonicalPath();
   }