Add ability to provide an anchor for the popover

[innocenzi]

Description

The popover uses the content of the default slot to create a trigger, but there is no way to provide a custom anchor. A <template #anchor> slot for this purpose would be great.

<UFormField label="name" name="name">
  <UPopover :open="isAutoCompleteOpen">
    <template #anchor>
      <UInput v-model="state.name" />
    </template>
    <template #content>
      <Placeholder class="size-48 m-4 inline-flex" />
    </template>
  </UPopover>
</UFormField>

My use case is building an autocomplete for a text input. The InputMenu is not suitable because the dropdown is too intrusive.

Additional context

No response