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

[zach]

Plugin::function_extists and extism_plugin_function_exists should check the signature of the functions to ensure they can be called by the Extism runtime.

A function is valid only if it takes no parameters and returns either an I32 or nothing.

[Onigbinde Oluwamuyiwa Elijah]

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

[zach]

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!

[Onigbinde Oluwamuyiwa Elijah]

Thanks @zshipko !

[zach]

Closed by #664