runtime: update `Plugin::function_extists` and `extism_plugin_function_exists` to check the type of the functions too

[zshipko]

runtime: update `Plugin::function_extists` and `extism_plugin_function_exists` to check the type of the functions too

[OLUWAMUYIWA]

First timer here. I'd like to pick this up.

[zshipko]

Awesome!

This would require updating function_exists to check the type of the params/results here instead of just checking is_some. Only functions that have no parameters and return either an I32 or nothing should be considered valid.

Let me know if you have any questions!

[OLUWAMUYIWA]

Thanks @zshipko !

[zshipko]

Closed by #664