[North] - Manage North Connectors with items (starting with Modbus)

[burgerni10]

Allow OIBus to write into Modbus register through a North Modbus Connector.

It should managed only points (see modes in north manifests)
It should be based on the South connector logics. It will need to introduce North Connector Items to map the pointIds (stored in value caches) to the Modbus Addresses.

I also suggest that each item store its last value written in a north_cache table (similar to south_cache), so we can make some logic based on that (for example, should we write the value again if it's the same than the one stored).

About the Modbus connection: some PLC has limited connections. We may later use the same connection between South and North Modbus connectors. Keep that in mind while developing this part, even if we can implement this shared connection later.

• Create OibusDataValue interface (@burgerni10)
• Create a item mode for North (@nagyszabi)
• Create HandleItemValues interface for North (@nagyszabi)
• Create north item data model and table / repository (Similar to South item repository, without scan mode) (@nagyszabi)
• Create Modbus North connector (@nagyszabi)
• Create the item section in the frontend (@burgerni10)
• Create Modbus North interface (@burgerni10)
• Implement the handleItemValues method for North modbus (See the read methods of south Modbus) (@nagyszabi)
• Create a North cache database (@nagyszabi)
• Add options to send values (compare to last value, last timestamp) (@nagyszabi)
• Suggest items name from north subscription (@burgerni10)