`secp256k1::SecretKey` is required in order to use the offline signing functionality but it's not re-exported. 

This is a simple PR that re-exports the `SecretKey` which allows the following:

```rust
let s = web3::SecretKey::from_str("XYZ");
web3.accounts().sign_transaction(my_tx, &prvk).await?;
```

without having to import `secp256k1` as an explicit dependency alongside `web3`.