signBundle doesn't set correct chainId when passed a FlashbotsBundleTransaction

signBundle() can take a mix of FlashbotsBundleTransactions and FlashbotsBundleRawTransactions. In the case of FlashbotsBundleTransaction, the user provides an (unsigned) TransactionRequest and a Signer. signBundle() populates certain fields of the transaction in case they're missing, like: nonce, gasPrice, gasLimit; however it doesn't include chainId, which ends up being 0 even on Görli.

To be fair, the [testnet example](https://docs.flashbots.net/flashbots-auction/searchers/advanced/goerli-testnet/) does explicitly set chainId to 5, however it could still make sense to automatically populate chainId, either using the Signer's getChainId() method or its populateTransaction(). Notably: when using a Contract's populateTransaction.XXX methods, chainId will be missing from the resulting object.