imu: make it part of InputState

Putting this here to see what CI has to say. Not super happy with this, input state seems to expand/copy too much

python_payload/st3m/power.py

+import machine
+import time
+
+
+class Power:
+    """
+    Collects information about the power state (e.g. battery voltage) of the badge.
+
+    Member of st3m.input.InputState, should be acquired from there, not instantiated manually.
+    """
+
+    def __init__(self) -> None:
+        self._adc_pin = machine.Pin(9, machine.Pin.IN)
+        self._adc = machine.ADC(self._adc_pin, atten=machine.ADC.ATTN_11DB)
+        self._battery_voltage = self._battery_voltage_sample()
+        self._ts = time.ticks_ms()
+
+    def _battery_voltage_sample(self) -> float:
+        return self._adc.read_uv() * 2 / 1e6
+
+    def _update(self) -> None:
+        ts = time.ticks_ms()
+        if ts >= self._ts + 1000:
+            # Sampling takes time, don't do it too often
+            self._battery_voltage = self._battery_voltage_sample()
+            self._ts = ts
+
+    @property
+    def battery_voltage(self) -> float:
+        self._update()
+        return self._battery_voltage