implement NIP-70: protected events

[fiatjaf]

a simple change can maybe fix the world.

proposal here: https://github.com/nostr-protocol/nips/pull/1030

(~~this code is broken and I don't know how to fix it, but hopefully it conveys the idea~~ the code was fixed by @v0l now works wonderfully)

[v0l]

Could add 70 here: https://github.com/hoytech/strfry/blob/c0dec7c53f82a72e41bc3e49f77b97bd9746bbb9/src/apps/relay/RelayWebsocket.cpp#L51

[hoytech]

Thanks!