feat(client): fallback to a hardcoded relay

[Nuhvi]

Add an option to use a hardcoded relay, this should allow Bitkit to not log relays in query params, and just focus on our server, which will make migration from "web relays" easier in the future after going out of beta.

[dzdidi]

Isn't opts.relay serving this purpose already?

[Nuhvi]

@dzdidi No, because that gets included in the URL when you call url() https://github.com/slashtags/web-relay/blob/main/lib/client/index.js#L76

[dzdidi]

I see, how about calling it defaultRelay (without _ prefix)? In my understanding used for internal things but this one is passed in options. Same applies for already implemented _skipCache, so IMO removing prefix there will make it better

[Nuhvi]

It is prefixed because I don't want anyone already using the library (if any) to worry about it. The whole thing will be deprecated soon anyway.