[inputs.modbus] Modbus UDP Feature Resquest

Feature Request

Add the ability to use Modbus/TCP over a UDP transport.

Proposal:

Add the ability to poll Modbus/TCP slaves over UDP controller = "udp://localhost:502"

Current behavior:

controller = "tcp://localhost:502" modbus plugin establishes a TCP connection to the modbus slaves and polls registers

Desired behavior:

controller = "udp://localhost:502" modbus plugin send modbus request over UDP to poll registers

Use case:

Modbus slaves that support UDP would benefit from the reduced overhead especially when telegraf isn't the only master polling the device for data. The same RTUoverTCP and ASCIIoverTCP options could remain.