anyOf support

[Geksanit]

I suggest adding support anyOf to codegen.
the difference between anyOf and oneOf is discussed here: https://stackoverflow.com/a/56039838

[kokovtsev]

That SO answer might be incorrect about mixed element types in arrays.

It appears that the difference between oneOf/anyOf only affects the validation:

• anyOf allows inputs that conform to multiple anyOf members;
• oneOf requires that the input object matches exactly one union member. After a match is found, we should check that the input does not match any other options.

The generated type would be a union type in both cases.

In fact, we do not do this "exactly one" check, so our implementation of oneOf actually behaves like anyOf.

[kokovtsev]

@Geksanit please review some work-in-progress in #181, if you're ok with the approach I will apply the same to OpenAPI 2.0 and AsyncAPI