Add uint64 & other 64bit support in Device Entries in Modbus Simulator

[schwarzkunstlerin]

Why there is no support in configuration for uint64 in Device Entries in Modbus Simulator? I think there should be one and we should be able to initialize this type of data stored in the simulator's register, especially since the library supports reading and writing uint64.

I'm currently working on simulating a device with u64int registers, using the JSON config and I really miss the types!