Add explicit gas check

[cam-schultz]

Why this should be merged

Adds an explicit check against the gas limit specified in the Teleporter message.

It's possible for the required gas limit in a Teleporter message to exceed the block gas limit. This causes the transaction to fail as expected, but also causes the relayer to enter an unhealthy state, preventing it from updating the checkpoint in its database. This can cause compounding latency that will eventually prevent the relayer from running at all.

How this works

Generally speaking, such validity checks are done in ShouldSendMessage and the Decider service. This is done before
signature aggregation, to short circuit that step if the message is invalid. However, the number of signers impacts the gas estimation, so we can't use that in ShouldSendMessage. Instead, I added a hard cap of 12 million gas, which is the C-Chain's 15 million gas block limit minus other Warp message related overhead that is conservatively estimated. Specifically, the 12 million figure assumes:

• 10kb Warp message size
• 8kb Teleporter message size
• 1000 signing validators
• 5 Teleporter receipts (the maximum amount)

How this was tested

CI, updated unit tests

How is this documented

N/A

[michaelkaplan13]

Makes sense to me, thanks for turning it around quickly 🙏

We may want to consider making the max required gas limit configurable per destination chain (since subnet-evm chains can set their own block gas limits), but not necessary for this PR.

[iansuvak]

LGTM and I agree that ShouldSendMessage is the right place for it for now.

If we come up with more cases like this we could group them together down the road into a separate helper method to distinguish between cases where the message is invalid due to relayer settings vs message validity like in this case.

[cam-schultz]

We may want to consider making the max required gas limit configurable per destination chain (since subnet-evm chains can set their own block gas limits), but not necessary for this PR.

Created an issue for this: #432