[infer] DefaultDiff - Make it easy to customize diff behavior without re-implementing diff

[iwahbe]

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Issue details

When infer based resources want to customize the Diff call, they can implement CustomDiff. This is great when the author wants full control of Diff's functionality, but it's expensive and bug-prone if the provider author only wants to tweak diff's behavior (example: #113).

Check & CustomCheck had the same problem, which was solved by the DefaultCheck function:

DefaultCheck is implemented so that if a resource r implements CustomCheck with DefaultCheck, they get the same behavior as not implementing CustomCheck at all:

func (r MyResource) Check(
	ctx context.Context, name string, oldInputs, newInputs resource.PropertyMap,
) (I, []p.CheckFailure, error) {
	return infer.DefaultCheck(ctx, name, oldInputs, newInputs)
}

By allowing custom implementations to wrap the default behavior, provider authors can make arbitrary tweaks to how Check works without the need for re-implementing existing behavior.

We should bring this same functionality to Diff & CustomDiff.

DefaultDiff should work for infer.Resource and infer based provider configuration.

Motivated by #113