This PR fixes an architectural problem we've seen with how to handle tags. I initially had an incomplete understanding of what a tag on Nostr is.

This change generalizes the Tag type to more closely match the Nostr spec. Each tag has a "name" and "value" and "other parameters", just like the README of the NIPs repo describes them. There will no longer be specific types for each name, because they cannot always be interpreted consistently outside the context of a specific event kind. We will still be able to have strongly typed and well-named properties at the event level.