feat(VDateInput): add slot for customizing VDatePicker actions

[yuya2017]

Description

This PR adds a new slot to the VDateInput component, allowing developers to customize the actions section of the VDatePicker. This enhancement provides greater flexibility in tailoring the date picker to specific application requirements. Resolves #20690.

Markup:

<template>
  <v-app>
    <v-container>
      <v-date-input>
        <template #actions>
          <v-btn text @click="cancel">cancel-custom</v-btn>
          <v-btn text @click="save">save-custom</v-btn>
        </template>
      </v-date-input>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
    setup () {
      return {
        cancel () {
          console.log('cancel')
        },
        save () {
          console.log('save')
        },
      }
    },
  }
</script>