Fix brittle sensor definition dictionary loading, and make things properly extensible

[AnotherDaniel]

At the moment, the sensor definitions used by the ha_mqtt plugin are are kept in a central file that contains dictionaries (src/utils/smasensors.py), which looks like this:

SENSORS_TRIPOWERX = [
    # device info
    {
        'key': "device_info.name",
        'name': "Device name",
        'entity_category': "diagnostic",
    },
   ...
]

This file contains definition dictionaries for all known devices (except for the new EV Charger plugin, all in the same style.

Right now, this is imported in ha_mqtt.py via: from utils.smasensors import * And then, I pick individual dictionaries from this file depending on context, with sensors_dict = f"SENSORS_{name.split('.')[0]}".upper() (The details don't matter - the essence is that this is selecting dictionaries from that larger file based on their name, at runtime)

There are at least two problems with this:

• This dictionary-set should be properly extensible by plugins, without touching the core smasensors.py script
• The approach is generally brittle, not very idiomatic, and not very intuitive to understand

So, this should be refactored, eg

• Use a Registry Pattern: Instead of separate dictionaries, have one dictionary that acts as a registry. New sensor definitions can be added to this registry.
• Use a Function to Register New Sensors: This is similar to the registry pattern but encapsulates the updating logic in a function.