fix(theme): Footer Column/Link should merge provided className

[slorber]

Motivation

We should be able to assign className to footer columns/items

The className should be merged to the default class instead of overriding it, for consistency with the behavior of navbar items, as suggested here: #10785 (comment)

{
      footer: {
        style: 'dark',
        links: [
          {
            title: 'Legal',
            className: 'footer-column-legal',
            items: [
              {
                label: 'Privacy',
                className: 'footer-item-privacy',
                href: 'https://opensource.facebook.com/legal/privacy/',
              },
            ],
          },
        ],
      },
}

Test Plan

CI + dogfood on preview site footer

Test links

https://deploy-preview-10796--docusaurus-2.netlify.app/

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🔴 35	🟢 98	🟢 96	🟢 100	Report
/docs/installation	🟠 50	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 73	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 63	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🔴 48	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 65	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟠 86	Report