Add parser support for CREATE loadable functions and DROP function

## Enhancement

```
CREATE [AGGREGATE] FUNCTION [IF NOT EXISTS] function_name
RETURNS {STRING|INTEGER|REAL|DECIMAL}
SONAME shared_library_name
```

```
DROP FUNCTION [IF EXISTS] function_name
```

ref https://dev.mysql.com/doc/refman/8.4/en/create-function-loadable.html

https://dev.mysql.com/doc/refman/8.4/en/drop-function-loadable.html

Use the linked MySQL CREATE FUNCTION and DROP FUNCTION references as the behavior specification. Start by locating TiDB's SQL parser grammar and related parser tests, then add coverage for the shown forms and verify that the supported syntax is accepted.

Written by the indexing model from the issue text.