Slot to change the default icon for the NTimePicker

[DannyFeliz]

This function solves the problem (这个功能解决的问题)

My team needs a custom icon for the time picker, and currently, there's no slot available to replace the default one.

Expected API (期望的 API)

<n-time-picker>
  <template #icon>
    <n-icon>
      <clock-icon />
    </n-icon>
  </template>
</n-time-picker>