Add ExtensionSigner, handle injected provider events

[nateReiners]

Summary

• Add ExtensionSigner which wraps the injected extension provider (window.coinbaseWalletExtension)
  • Eventing
  • Handles "chainChanged" and "accountsChanged" events emitted by extension provider
  • Updates CoinbaseWalletProvider state accordingly.
• Remove getCoinbaseInjectedProvider method and inline the logic since its only used in one place
• Remove unused injected signer logic
• Tested with unit tests and local test:

https://github.com/coinbase/coinbase-wallet-sdk/assets/18184751/ca8b9186-f188-40af-91dc-1fdacb75c51a

[nateReiners]

Will need to add chain and accounts getters as a followup