This is an experimental feature, outside of any NIP, that demonstrates
generic tag queries.
Instead of limiting subscription filters to just querying only "e" or
"p" tags (via `#e` or `#p` attributes), any tag can be queried.
As an example, consider an event which uses a tag "url". With this
modification, a subscription filter could add a top-level field
"#url", with an array of strings as the key. Exact matches would be
returned.
A NIP is forthcoming to formalize this.
The `authors` field is sufficient to represent all queries that
`author` could have been used in. See
https://github.com/fiatjaf/nostr/issues/34 for the discussion leading
to this removal.
