[WIP]Make the FTP extension coroutine-enabled

[NathanFreeman]

Hello everyone, I plan to add coroutine support for FTP in Swoole. However, in the current FTP implementation, the use of poll is hardcoded, which lacks flexibility and makes it impossible to coroutine-enable FTP in Swoole. Therefore, I have made the following modifications to the my_poll function in ftp.c:

1. Replaced the direct call to php_pollfd_for_ms with a function pointer named php_ftp_pollfd_for_ms.
2. During the extension initialization phase, the pointer is set to point to the original php_pollfd_for_ms implementation by default, ensuring the existing synchronous logic remains unaffected.
3. When running in a coroutine environment, this function pointer can be replaced with a coroutine-compatible polling function (such as a non-blocking version based on an event loop), thereby achieving non-blocking FTP socket operations for the main thread.

This modification does not affect the original FTP logic and introduces no syntactic changes.

With this change, I can easily assign a value to php_ftp_pollfd_for_ms in my own extension, enabling coroutine support for FTP without any disruptive intrusions into the FTP code.

swoole pr：swoole/swoole-src#5899

[NathanFreeman]

But I'm not very familiar with the PHP compilation process on Windows and have encountered the following issue. Could this be due to a step I might have missed? Thank you!

ftp.obj : error LNK2019: unresolved external symbol __imp_php_ftp_pollfd_for_ms referenced in function my_poll
php_ftp.obj : error LNK2001: unresolved external symbol __imp_php_ftp_pollfd_for_ms
C:\obj\Release_TS\php_ftp.dll : fatal error LNK1120: 1 unresolved externals

windows error: https://github.com/php/php-src/actions/runs/18814180127/job/53680051090?pr=20299

[kallesommernielsen]

@NathanFreeman take a look at ext/enchant, specifically PHP_ENCHANT_API should be an example of how an extension can export its symbols. Hope it helps

[NathanFreeman]

@kallesommernielsen Thank you, I'll give it a try.