Support for fallback hubUrls

[ggomaeng]

It would be pretty neat to pass in an array of hubUrls like Viem's fallback.

There are times when the hub is down and it just makes the app non-functional.

Current

import { Frog } from 'frog'
 
const app = new Frog({
  hub: {
    apiUrl: 'https://api.hub.wevm.dev'
  }
})

Proposal

import { Frog } from 'frog'
 
const app = new Frog({
  hub: {
    apiUrl: ['https://api.hub.wevm.dev', 'https://freefarcasterhub.com', ...]
  }
})

I used to do something like below before finding frog 🐸

https://gist.github.com/ggomaeng/7895a2e410874ca87f5534890055d20c

[dalechyn]

Moved to issue #488