feat: custom column generator

[andreatgretel]

Adding a CustomColumnGenerator that works essentially the same as the LocalCallableValidator while being more generic.

Still a few points to address:

• what if the user wants to call an LLM inside of the generator function?
• should we support strategies other than GenerationStrategy.FULL_COLUMN?
• should we support multiple columns being added at once?

Closes #6.

[andreatgretel]

This will be removed, just wanted to add an example script to test the PR somewhere.

At the same time we should maybe think of having such a folder somewhere?

[johnnygreco]

If we did something like

result = self.config.generator_function(data, resource_provider=self.resource_provider)

the user could then optionally leverage LLMs and other resources in their custom generate method. I'm exploring something like this in the plugin decorator.

[andreatgretel]

I'm thinking what's the best way of doing this... For calling LLMs in particular, it should probably also leverage the remaining of what's in place - threading etc.
Should it inherit WithLLMGeneration perhaps? 🤔

Also LLM generators are typically CELL_BY_CELL... Not clear to me how to specify the generation strategy here.

[johnnygreco]

Oh yeah, good point. Maybe we should split that mixin up a bit so that we can just get the useful properties (and not the shared generate method)

We could let the user choose the generation strategy? Haven't thought about how messy that would make the implementation, but it would basically define the signature of their generate method.