fix: make GatewayContract compatible with current KMS in trustless case

[jatZama]

This PR makes the code of GatewayContract and Gateway library a lot more complex than previously, but this was needed since KMS did not implement the specific data handling in the trustless decryption case, and we decided not to change anything from KMS side. Since the trustless case became more complex, it consumes more gas, but we had no other option since we want the KMS to be agnostic to wether decryption was requested in trustless or trusted mode.

To mitigate the increase in gas cost, I implementend one of the optimizations we talked about : I am no longer doing a KMSVerifier call inside the GatewayContract in the trustless case (since in this case we assume that the dapp will do the verification itself).

Some other more minor improvements include : switching the external modifier of removeOffset to internal, and moving the $.isFulfilled[requestID] = true; line at the start of the fulfillRequest function as this will be needed to avoid reentrancy later, because in a future version we will probably remove the onlyRelayer modifier, to avoid "relayer censorship" risk.

Once you approve the PR, I will need to release the patched version of fhevm-core-contracts to also upgrade the gateway library similarly in fhevm and also release then a patch of fhevm fyi.

[jatZama]

This closes #180