add sql server drop index rule, drop table rule, truncate rule

sharding-jdbc-ddl-parser/src/main/antlr4/io/shardingsphere/parser/antlr/sqlserver/SQLServerAlterIndex.g4

 grammar SQLServerAlterIndex;
-import SQLServerKeyword, DataType, Keyword, SQLServerBase,BaseRule,Symbol;
+import SQLServerKeyword, DataType, Keyword, SQLServerBase, BaseRule, Symbol;
 
 alterIndex:
 	alterIndexOp

sharding-jdbc-ddl-parser/src/main/antlr4/io/shardingsphere/parser/antlr/sqlserver/SQLServerDropIndex.g4

+grammar SQLServerDropIndex;
+import SQLServerKeyword, DataType, Keyword, SQLServerBase, BaseRule, Symbol;
+
+dropIndex:
+    DROP INDEX tableOrViewName (WITH LEFT_PAREN dropIndexOptions RIGHT_PAREN)?
+    ;
+
+dropIndexOptions:
+    dropIndexOption (COMMA dropIndexOption)*
+    ;
+
+dropIndexOption:
+    MAXDOP EQ_OR_ASSIGN NUMBER
+    | ONLINE EQ_OR_ASSIGN (ON | OFF)
+    ;
\ No newline at end of file

sharding-jdbc-ddl-parser/src/main/antlr4/io/shardingsphere/parser/antlr/sqlserver/SQLServerDropTable.g4

+grammar SQLServerDropTable;
+import SQLServerKeyword, DataType, Keyword, SQLServerBase, BaseRule, Symbol;
+
+dropTable:
+    DROP TABLE (IF EXISTS)? tableNames
+    ;
\ No newline at end of file

sharding-jdbc-ddl-parser/src/main/antlr4/io/shardingsphere/parser/antlr/sqlserver/SQLServerTruncate.g4

+grammar SQLServerTruncate;
+import SQLServerKeyword, DataType, Keyword, SQLServerBase, BaseRule, Symbol;
+
+truncate:
+    TRUNCATE TABLE tableName (WITH LEFT_PAREN PARTITIONS LEFT_PAREN partitionExpressions RIGHT_PAREN RIGHT_PAREN)?
+    ;
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