feat(VDataTable): add cellClass property to headers

[peteruithoven]

Description

DataTable headers cellClass property enabling adding classes to td's

fix #8474

Motivation and Context

It's hard to add classes to DataTable cells / td's. Being able to add these classes more easilly enables:

• Tweaking width of columns when the header is hidden (hide-default-header)
• Changing the padding in the cells
• Other kinds of styling that would otherwise require a extra element like setting a background or text color.

Also see: #8474

How Has This Been Tested?

I've tested this in the Playground and I've added a unit test:
https://github.com/vuetifyjs/vuetify/pull/11775/files#diff-44a9b747492d2b82ba824229ad9e5800

Markup:

<template>
  <v-container>
    <v-data-table
      :headers="headers"
      :items="items"
    />
  </v-container>
</template>

<script>
  export default {
    data () {
      return {
        headers: [
          { text: 'Impact', value: 'name', cellClass: 'pl-0' },
          { text: 'Score', value: 'score', align: 'end', cellClass: ['pt-8', 'pb-8'] },
          { text: 'Counter', value: 'counter', align: 'end' },
        ],
        items: [
          { name: 'A', score: 10, counter: 1 },
          { name: 'B', score: 20, counter: 2 },
          { name: 'C', score: 30, counter: 3 },
        ],
      }
    },
  }
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)

I'm not sure what to add to the documentation? I did update the API.

[ibrahimBeladi]

@nekosaur
Could you review this pull request?

[milakucher]

it's been a while, but thanks for the cellClass anyway! :)