Inaccuracy in Approve Chain Extension behavior

[ashneverdawn]

While implementing the weight charging for the token allowance pallet, I noticed that the approve behavior isn't consistent with OpenZeppelin's one.

1) The current behavior increments the approval, whereas it should overwrite.

See Docs: https://docs.openzeppelin.com/contracts/4.x/api/token/erc20#IERC20-approve-address-uint256-

Current behavior: https://github.com/pendulum-chain/pendulum/blob/0f6cbb5289d03bd85ab664e7724aba691fea8d81/pallets/orml-currencies-allowance-extension/src/lib.rs#L208C3-L208C3

Expected behavior: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/21bb89ef5bfc789b9333eb05e3ba2b7b284ac77c/contracts/token/ERC20/ERC20.sol#L345
Fixing this should also result in a lower gas cost when regenerating the weights. (no longer need to read before writing)

2) When approved for the max possible amount, transfer_from should treat it as infinity and not decrement it.

See Docs: https://docs.openzeppelin.com/contracts/4.x/api/token/erc20#ERC20-approve-address-uint256-

[ebma]

Hey team! Please add your planning poker estimate with Zenhub @ashneverdawn @TorstenStueber