refactor: rename `RelayURL` to `DefaultRelayURL`

It's the case after !87 (merged) Now the broker specifies the relay URL (see ProxyPollResponse).

• This is gonna be a breaking change for library users, they'll have to update the name in case they're using the RelayURL property. Is it fine?

FYI this will conflict with !370 (merged) but only on one line.