[R4R]: Migrate some methods from universal to implements

[Ethanncnm]

Base on original optimism contract, we just change some methods in StandardBridge and CrossDomainMessage to virtual, and override these methods in L1StandardBridge, L2StandardBridge, L1CrossDomainMessage and L2CrossDomainMessage.

In later PRs, we just need to update these override methods, thus we won't be confused about what is msg.value represents to. In L1StandardBridge and L1CrossDomainMessage, msg.value represents ETH token. In L2StandardBridge and L2CrossDomainMessage, msg.value represents MNT token.

[Sha3nS]

migrate some method from universal to implements
LGTM