feat: Add providers.xmpp.net to AccountCreationWizard

Closes #10593 (closed)

What this does:

When clicking "Sign Up" in the account wizard, Gajim downloads the latest providers list from https://data.xmpp.net/providers/v2/providers-B.json

This list is then presented to the user, providing suggestions for which provider to choose. No provider is selected by default.