GasEstimator issue triggered by changing U128 serialization

[benesjan]

In this PR I've changed U128 serialization such that it serializes to 2 fields and not 1. This then makes it possible to use as contract function arg as the resulting serialization format matches intrinsic serialization of Noir (U128 struct contains 2 limbs and that's what Noir assigns the values to when matching the witness with the function args).

But for whatever reason this has caused gas_estimation.test.ts to fail (specifically the second and third test case).

Palla helped me with debugging and he thinks this is an issue with the estimator. There is a workaround - if I set estimatedGasPadding to a larger value the tests passes.

The plan is that I will revisit this once I implement Packable trait because once that is done U128 will be once again stored as 1 field in storage. This will help us determine whether the issue is storage related as the storage costs should get to values before this PR was made.

Look for "TODO(#11324)" in the codebase to find where the workaround was placed.

Here are gas estimation values I obtained for different paddings in the estimates gas with public payment method test case and info whether the test passed (F - fail, P -pass):

F padding 0: Estimated gas limits for tx: DA=8192 L2=454967 teardownDA=2048 teardownL2=97792
F padding 0.0005859375: Estimated gas limits for tx: DA=8197 L2=455234 teardownDA=2050 teardownL2=97850
P padding 0.00068359375: Estimated gas limits for tx: DA=8198 L2=455279 teardownDA=2050 teardownL2=97859

Saving them here as they might be useful later on.