feat(eth): add w3.eth.sign_authorization() EIP-7702 convenience helper

[kushdab]

Addresses #3655 — auto-nonce/chainId bare-metal improvement.

Motivation

The current EIP-7702 workflow forces callers to manually fetch chain
state before every authorization:

signed_auth = acct.sign_authorization({
    "chainId": w3.eth.chain_id,                          # manual
    "address": contract.address,
    "nonce": w3.eth.get_transaction_count(acct.address), # manual
}, private_key)

As @wolovim noted, web3.py can provide "minor niceties, like auto nonce
management in the bare metal versions." This PR is that niceity.

API

# Minimal — chainId and nonce auto-filled from chain state
auth = w3.eth.sign_authorization(contract.address, my_private_key)

# Opt out of auto-fill for either field
auth = w3.eth.sign_authorization(
    contract.address,
    my_private_key,
    chain_id=0,    # chain-agnostic (replayable anywhere)
    nonce=nonce+1, # explicit nonce, e.g. when EOA submits its own type-4 tx first
)

The result is a SignedSetCodeAuthorization ready for authorizationList:

tx = {
    "from": acct.address,
    "to": acct.address,
    "authorizationList": [auth],
    "data": contract.encode_abi("myFunction", args=[arg1, arg2]),
}
signed = acct.sign_transaction(tx)
w3.eth.send_raw_transaction(signed.raw_transaction)

Changes

File	Change
web3/eth/eth.py	Add Eth.sign_authorization(address, private_key, *, chain_id, nonce)
web3/eth/async_eth.py	Add AsyncEth.sign_authorization(...) (awaitable)
tests/core/eth-module/test_eth_sign_authorization.py	15 tests covering auto-fill, explicit overrides, nonce/chainId short-circuit (RPC not called when explicit), and structural fitness as an authorizationList item