WIP: force one of flag

[fmmoret]

Hey, I am working with an API that I cannot add discriminators to, but downstream generators can't consume AnyOf properly in some cases.

Here are some similar threads:
#162
#163

This PR is a proposal for an interface to let a user opt into a potentially unsafe representation of their schema.

If the direction is approved, I can add testing / ensure correctness of my implementation -- I have not manually checked it yet for correctness, but want feedback on the interface & approach.

[fmmoret]

cc @AGalabov

[AGalabov]

@fmmoret Thank you for opening a PR and sorry for taking long on this. I think I do agree that we can add such an option (as opposed to the previous implementation in #163).

A couple of pointers:

1. I think it makes sense to be consistent across the document => I would rather move the configuration at constructor level instead of through the .openapi method. I simply cannot envision why one would want to have two ZodUnion schemas with different signature within a single document.
2. We tend to add tests for all features/bugs that we handle. Please add some for this.
3. I see some checks are failing - please take a look at them

I'd also cc: @georgyangelov to share his opinion on the matter.

[georgyangelov]

@fmmoret thank you for the contribution.

@AGalabov I'm a bit on the fence for constructor parameter vs .openapi. On one hand the .openapi approach gives more control - you can use oneOf for some of the types (I also can't think of a reason to want that but that doesn't mean there isn't any). On the other hand - that way would mean going through all of your types and adding that to every single one where a union may be declared. The latter is I believe more of an issue so I support the global switch in the geenrator object approach - that would mean users won't forget to add the flag when the schemas change.