Optionally define unique sensor id in inverter response decoder

[niclasku]

This is a fix for #112. It allows us to optionally define the unique sensor used by Home Assistant in the inverter's response decoder:

"Run Mode Raw": {
    "decoder": (21, Units.NONE),
    "unique_id": 200,
},

I believe explicitly defining the id is better than generating an id. An id generation algorithm would always depend on some value or order of a list, tuple or dict so that there is no guarantee that the generated id will not change in the future.

@squishykid How do you think about this solution? Let me know if you have a better idea!