contracts-bedrock: add some view functions to FaultDisputeGameN

[JustXxx]

To support op-challenger's functionality, this PR slightly modified FaultDisputeGameN.sol:

• add nBits and maxAttackBranch view functions in FaultDisputeGameN
• modify the attack branch variable to uint256, to unify with other variable types

Regression tests

forge test --mc FaultDisputeGameN_Test forge test --mc FaultDisputeN_1v1_Actors_Test

[dajuguan]

Why attack branch variable's type need to be unify with other variable types?

[JustXxx]

Why attack branch variable's type need to be unify with other variable types?

When calling in Go language, there is no need to handle different data types.