Add max gas limit to config

[cam-schultz]

Context and scope
#430 added a hardcoded gas limit to prevent invalid transactions from being sent. That hardcoded limit is based on the 15 million block gas cap for the C-Chain. However, subnet-evm chains can define their own gas limit, so it would make sense to make this configurable.

Open questions
Where should this live in the config? RequiredGasLimit is a TeleporterMessage field, so SourceBlockchain.MessageContracts would fit there. OTOH, the gas limit is determined by the destination chain, so DestinationBlockchain is also an option.