Allow PubkyClient and Homeserver to override default pkarr relay

[catch-21]

// Initialize PubkyClient with Pkarr relay(s).
let client = new PubkyClient();

This currently forces use of https://relay.pkarr.org
used by pubky-app: https://github.com/pubky/pubky-core/blob/main/pubky/src/wasm.rs#L31

Homeserver is also using a hardcoded default: https://github.com/pubky/pubky-core/blob/main/pubky-homeserver/src/server.rs#L42, which is defined: https://github.com/pubky/pkarr/blob/main/pkarr/src/lib.rs#L32

It would be useful, for the automated tests at least, to allow pubky-app and the homeserver to be configured with a locally deployed pkarr relay.