feat: add option to pass client url when initializing relayer channel

[ahollenbach]

[draft to explain the concept, not a fully baked PR]

Motivation

As it currently exists, the only client supported by the Warpcast relay servers is the Warpcast app, redirecting all auth requests to https://warpcast.com/~/sign-in-with-farcaster. Apps like Supercast currently have no way to have other apps SIWF unless they export a seed phrase and import that into the Warpcast app.

This PR adds some light flexibility to this by allowing the implementing page to specify the target client URL

This PR does not address any larger issues around assembling a registry or require handling. It currently does not recommend any changes to AuthKit itself - implementation of passing in the client URL is up to individual implementers to start.

Change Summary

Instead of pulling from env, channel creation is updated to support passing in a client URL when initializing the channel. An example of how this could be used:

function initSignInWithFarcaster(clientUrl?: string) {
  const res = await axios.post('https://relay.farcaster.xyz/v1/channel', {
      ...,
      clientUrl
  });
  // 
  console.log(res.data.connectUri)
  // => https://api.supercast.xyz/siwf?channelToken=...
}

...

<button onClick={() => initSignInWithFarcaster()}>Sign in with Warpcast</button>
<button onClick={() => initSignInWithFarcaster("https://api.supercast.xyz/siwf"}>Sign in with Supercast</button>

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