Represent unlimited ICS20 authorization amount as MaxInt256

[Vvaradinov]

Summary

ICS20 authz spending limit always subtracts even if the value represents the maximum amount.

Problem Definition

The new ICS20 authorization included in v7 takes in a spending limit as a list of sdk.Coin and type int64. At Evmos we use ERC20 token allowance approvals that support an unlimited amount (which are represented as the max uint256 value). If we pass in the max uint256 amount effectively making it unlimited to the ICS20 authorization we assume that the amount will not be subtracted on each transfer.

Proposal

We propose the unlimited value be represented as the MaxUint256. This will require an additional check and skip substracting from the SpendLimit if the amount is MaxUint256.

var MaxUint256 = new(big.Int).Sub(new(big.Int).Lsh(big.NewInt(1), 256), big.NewInt(1))
if allocation.SpendLimit.AmountOf(msgTransfer.Token.Denom).Equal(sdk.NewIntFromBigInt(MaxUint256)) {
	return authz.AcceptResponse{Accept: true, Delete: false, Updated: &a}, nil
}

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