Add validation events decorators

[sugmanue]

Description of changes:

Adds a new interface ValidationEventDecorator that libraries can implement and plug into Smithy by making those discoverable using the java builtin ServiceLoader. All of the ValidationEventDecorator instances are loaded and each ValidationEvent is passed to each of the decorators. The decorators can make use of the ValidationEvent#eventId() method to quickly filter out the events that it knows how to decorate form the events it does not care about, and the ValidationEvent#toBuilder() to create a builder that can be used to mutate the instance.

The current use case is to add hints to a know set of events to point the user into the right solution for it (e.g., you need to pull X or Y dependency for this unresolved shape). Since often these solutions might require specific details about the environment in which Smithy is being used, those decorators cannot be part of Smithy itself but now can be implemented by libraries and loaded automatically by Smithy in a similar way in which validators are currently loaded.

For instance, a decorator that suggest the user to use or remove unused structures might look like this

    @Override
    public ValidationEvent decorate(ValidationEvent ev) {
        if (ev.containsId("UnreferencedShape")) {
            if (ev.getMessage().contains("The structure ")) {
                return ev.toBuilder()
                    .hint("Consider using this structure in your service or remove it from the model")
                    .build();
            }
        }
        return ev;
    }

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sugmanue]

Approved offline after the last requested change to change loadBuiltinDecorators to loadDecorators.