ValidationEventDecorator: Allow access to the Model

[kubukoz]

Currently, ValidationEventDecorator are pretty limited in what they can do to decorate events - all they receive is a ValidationEvent.

Having access to the Model, just like a validator would, would make it much easier to do interesting things.

For example, one might want to write a decorator that reads metadata; said metadata could contain user-definable explanations for some common TraitTarget selectors.

[mtdowling]

Can you pass a Model into the constructor of your decorator?

[kubukoz]

Not if I want it to be created with SPI - and I think that's the only way I can make the decorator work in LSP/smithy-cli contexts :/