feat: Add flag to fetch user relays

[0xtrr]

I think it would make it easier to e.g. delete events if we just fetched all user relays. Should probably be exposed as it's own flag at the root level of the command. So instead of doing nostr-tool -r wss://nostr.oxtr.dev -p {PRIVATE_KEY} delete-event -e {EVENT_ID} -r "The reason for deleting the event" we could do: nostr-tool -ur -p {PRIVATE_KEY} delete-event -e {EVENT_ID} -r "The reason for deleting the event" where the -ur flag is short for "user relays" or something similar.