[Linter] Check PodSpecs for unknown keys

[fabiopelosin]

As PodSpecs can by loaded by JSON or YAML files it would be useful to highlight any non supported key.

This should be implemented in a dedicated method in https://github.com/CocoaPods/Core/blob/master/lib/cocoapods-core/specification/linter/analyzer.rb#L6. The implementation could check the Pod::Specification#attributes_hashagainst the Pod::Specification::DSL.attributes. Each attribute agains the Pod::Specification::DSL::Attribute#name which is used as a key for the hash storing them

[alloy]

YES! Can we get this ASAP? (For trunk.)

[fabiopelosin]

This should also check for the type (the class) of values stored for each attribute.

[orta]

I wonder if we should allow a _ prefix that allows people to have spec metadata but only for plugin authors to work with?

[fabiopelosin]

@orta sounds like a good idea, but I would prefer a specific need to arise before implementing it.