feat: add signature function to TxDeposit (#174)

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## Motivation
`optimism_deposit_tx_signature` function in `reth` should be moved to
`op-alloy` as part of an
[issue](paradigmxyz/reth#11781).

paradigmxyz/reth#11781

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## Solution
An associated function `signature` is added to the `TxDeposit` struct.

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## PR Checklist

- [ ] Added Tests
- [ ] Added Documentation
- [ ] Breaking changes

.github/workflows/ci.yml

@@ -138,7 +138,7 @@ jobs:
           cache-on-failure: true
       - run: cargo +stable clippy --workspace --all-targets --all-features
         env:
-          RUSTFLAGS: -Dwarnings
+          RUSTFLAGS: -D warnings
 
   docs:
     runs-on: ubuntu-latest

crates/consensus/src/transaction/deposit.rs

@@ -1,7 +1,7 @@
 use super::OpTxType;
 use alloy_consensus::Transaction;
 use alloy_eips::eip2930::AccessList;
-use alloy_primitives::{Address, Bytes, ChainId, TxKind, B256, U256};
+use alloy_primitives::{Address, Bytes, ChainId, Parity, Signature, TxKind, B256, U256};
 use alloy_rlp::{
     Buf, BufMut, Decodable, Encodable, Error as DecodeError, Header, EMPTY_STRING_CODE,
 };
@@ -152,6 +152,12 @@ impl TxDeposit {
             inner_payload_length
         }
     }
+
+    /// Returns the signature for the optimism deposit transactions, which don't include a
+    /// signature.
+    pub fn signature() -> Signature {
+        Signature::new(U256::ZERO, U256::ZERO, Parity::Parity(false))
+    }
 }
 
 impl Transaction for TxDeposit {