CVE-2021-41138: Validity check missing in Frontier

In the newly introduced signed Frontier-specific extrinsic for pallet-ethereum, a large part of transaction validation logic was only called in transaction pool validation, but not in block execution. Malicious validators can take advantage of this to put invalid transactions into a block.

The attack is limited in that the signature is always validated, and the majority of the validation is done again in the subsequent pallet-evm execution logic. However, do note that a chain ID replay attack was possible. In addition, spamming attacks are of main concerns, while they are limited by Substrate block size limits and other factors.