Parameterize block gas limit

[cam-schultz]

Context and scope
https://github.com/ava-labs/awm-relayer/pull/430/files added a hardcoded tx limit of 12 million gas to comply with the C-Chain's 15 million gas limit per block. This limit may vary from subnet to subnet, so we should make this configurable.

Discussion and alternatives
We must add validation checks to cap the gas limit for C-Chain transactions, since this is a known (fixed) block limit.

Open questions