Added important note about optimism for auth addresses

[zacharytyhacz]

I was banging my head on my desk for hours figuring out why my auth address method was NOT working, only to learn that the RPC urls MUST BE OPTIMISM.

This was no where mentioned in the docs, so thought I help everyone out

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PR-Codex overview

This PR introduces a new appClient configuration for verifying sign-in messages using the @farcaster/auth-client. It sets up the necessary rpcUrls for the Optimism network.

Detailed summary

• Added createAppClient and viemConnector imports from @farcaster/auth-client.
• Configured appClient with a relay URL and Optimism rpcUrls.
• Implemented verifySignInMessage method with parameters nonce and domain.

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