Support Spark DROP TABLE command (unitycatalog#295)

**Description of changes**

<!-- Please state what you've changed and how it might affect the users.
-->

Support Spark DROP TABLE command

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

@@ -55,7 +55,7 @@ class UCSingleCatalog extends TableCatalog {
   }
   override def alterTable(ident: Identifier, changes: TableChange*): Table = ???
 
-  override def dropTable(ident: Identifier): Boolean = ???
+  override def dropTable(ident: Identifier): Boolean = deltaCatalog.dropTable(ident)
 
   override def renameTable(oldIdent: Identifier, newIdent: Identifier): Unit = ???
 }
@@ -93,9 +93,8 @@ private class UCProxy extends TableCatalog {
   }
 
   override def listTables(namespace: Array[String]): Array[Identifier] = {
-    if (namespace.length > 1) {
-      throw new ApiException("Nested namespaces are not supported:  " + namespace.mkString("."))
-    }
+    checkUnsupportedNestedNamespace(namespace)
+
     val catalogName = this.name
     val schemaName = namespace.head
     val maxResults = 0
@@ -174,8 +173,18 @@ private class UCProxy extends TableCatalog {
 
   override def alterTable(ident: Identifier, changes: TableChange*): Table = ???
 
-  override def dropTable(ident: Identifier): Boolean = ???
+  override def dropTable(ident: Identifier): Boolean = {
+    checkUnsupportedNestedNamespace(ident.namespace())
+    val ret =
+      tablesApi.deleteTable(Seq(this.name, ident.namespace()(0), ident.name()).mkString("."))
+    if (ret == 200) true else false
+  }
 
   override def renameTable(oldIdent: Identifier, newIdent: Identifier): Unit = ???
 
+  private def checkUnsupportedNestedNamespace(namespace: Array[String]): Unit = {
+    if (namespace.length > 1) {
+      throw new ApiException("Nested namespaces are not supported:  " + namespace.mkString("."))
+    }
+  }
 }

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

@@ -161,6 +161,20 @@ public void testShowTables() throws ApiException, IOException {
     session.stop();
   }
 
+  @Test
+  public void testDropTable() throws ApiException, IOException {
+    createCommonResources();
+    SparkSession session = createSparkSessionWithCatalogs(SPARK_CATALOG);
+    setupExternalParquetTable(PARQUET_TABLE, new ArrayList<>(0));
+    String fullName = String.join(".", SPARK_CATALOG, SCHEMA_NAME, PARQUET_TABLE);
+    assertTrue(session.catalog().tableExists(fullName));
+    session.sql("DROP TABLE " + fullName).collect();
+    assertFalse(session.catalog().tableExists(fullName));
+    AnalysisException exception =
+        assertThrows(AnalysisException.class, () -> session.sql("DROP TABLE a.b.c.d").collect());
+    session.stop();
+  }
+
   private String generateTableLocation(String catalogName, String tableName) throws IOException {
     return new File(new File(dataDir, catalogName), tableName).getCanonicalPath();
   }