Allowing using server managed Wallets like Privy/Coinbase to sign transactions

[anuraj-rp]

agent0_sdk requires a raw private key (or eth_account.BaseAccount) for all write operations.

The signing path in web3_client.py calls self.w3.eth.account.sign_transaction(tx, self.account.key) which needs direct key
access. This makes it impossible to use server-managed wallets (Privy, Coinbase CDP, AWS KMS, Fireblocks)

I could submit a PR for this in a backward compatible way.

Let me know what you think