Check `foo` implementation do support all `x-foo-*` extension fields

[ndeloof]

With extension field support, it's easy to add extension but also hard to report errors to user. Typically, we can't catch a typo.

I'd like we introduce a mechanism so that Compose implementation can declare extension it allows, and we can catch:

x-foo-unknown: bar

Would be awesome if such a mechanism can be used to define types, so that we don't just use interface{} for extensions.