[BUG] Custom chain icon file replacement is not supported.

[Pszz]

Describe the bug
Connectkit does not support custom chain icon file replacement.
When I link network and switch network, its icon is unknown.

import { defineChain } from 'viem'

// Custom chain
export const customTestChain = defineChain({
  id: 888888888,
  name: 'TestChain',
  testnet: true,
  iconUrl: '/imgs/chains/test_chain.svg',
  // ...some other code
})

^^ This method is better, especially for some private chains, which may not necessarily submit their logos to the public area.

// rainbowkit they can support.
"@rainbow-me/rainbowkit": "^2.1.2",