Validate RPC methods should not be final

[longquanzheng]

Context
Otherwise byteBuddy won't work when intercepting the call.

All method in Java by default is not final, but for Kotlin, open must be added for the RPC method to make it not final.

See https://stackoverflow.com/questions/54043234/bytebuddy-method-interception-doesnt-work-in-kotlin

Goal
We should throw error when the rpcStub is created or workflow is registered (prefer former). Because when invoking later, there is not obivous errors for this issues.