[Bug] doris antlr4 g4文件语法支持不全，没跟官方文档保持一致

[zheng594]

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• I had searched in the issues and found no similar issues.

Version

2.1 & master

What's Wrong?

doris源码中的antlr4 g4文件中的语法不全，常用的很多语法源码g4文件都不支持。
g4文件地址：/doris-master/fe/fe-core/src/main/antlr4/org/apache/doris/nereids/DorisParser.g4

不支持的示例语法：
ALTER TABLE example_db.my_table ADD COLUMN new_col INT KEY DEFAULT "0" AFTER key_1;
SHOW CREATE TABLE tab;
DESC tab;
DROP TABLE tab

What You Expected?

能像starrocks一样支持全部语法，跟官网文档保持一致

How to Reproduce?

No response

Anything Else?

No response

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct