For mysql_fdw. · Pull Request !20 · openGauss/openGauss-server

**What this PR does / why we need it**:

For mysql_fdw. Including mysql_fdw source code(https://github.com/EnterpriseDB/mysql_fdw) and patch.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:
1. Remove limit of 'create extension/create function/create foreign table/create foreign data wrapper/create foreign server/create user mapping'.
2. To make mysql_fdw, add '--enable-mysql-fdw' when configure. Test with maraidb, version 'mariadb  Ver 15.1 Distrib 10.5.4-MariaDB, for Linux (x86_64) using readline 5.1'
3. Patch for mysql_fdw:
        a) use mariadb as header file and lib, not mysql
        b) fix some compiler warning, most are type cast problem
        c) since we use g++ as compiler, add "C" in some extern symbol
        d) add THR_LOCAL for global var
        e) move mysql_fdw_exit to mysql_get_connection
        f) move function symbol from mysql_fdw.h to mysql_fdw.c to solve multi-definition problem
        g) check whether the guc option is already exist before call DefineCustomIntVariable
        h) fix input parameter problem for add_path
        i) use rte->updatedCols instead of rte->modifiedCols
4. Don't support 'drop extension' for now.

**Release note**:

```release-note
```

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