Add account management utility, which holds the owned accounts and private keys and can sign transactions. A similar utility is being used in the revive RPC. See if we can reuse and extend that, or if we can inject our own account manager into the revive RPC (with a common interface). Also implement the singing RPCs: eth_sign, eth_signTypedData_v4, eth_signTypedData, eth_signTypedData_v3, eth_signTransaction, personal_sign.