Suggestion: Use string for Additional Custom Link Attributes

[babs20]

I noticed the addition of types for custom link attributes which is great, but was thinking we shouldn't just use any as the value for additional properties. If the user does add additional properties these would have to be a string AFAIK from my testing of this feature with Storyblok's Content Delivery API. If you don't want to be too explicit, using an unknown would give better type safety.

I don't mind making a PR for this if it is something we want to go forward with.