feat: map user social meta fields

[colis]

What

This PR maps the social meta fields of the User object to the RankMathUserSeo type, i.e.

user(id: "5", idType: DATABASE_ID) {
  seo {
    ... on RankMathUserSeo {
      social {
        additionalProfiles
        facebookProfileUrl
        twitterUserName
      }
    }
  }
}

Why

The Rank Math plugin allows us to add user's Facebook URL, Twitter username and other profiles. Therefore this data needs to be mapped to the GraphQL schema.

How

Testing Instructions

1. Populate the aforementioned fields on the User profile page
2. Use the query above in the GraphiQL IDE

Additional Info

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [x] I have added unit tests to verify the code works as intended.
• [ ] The changes in this PR have been noted in CHANGELOG.md

[colis]

Code looks great - thanks @colis !

From a schema perspective, I'm wondering if we really want to introduce the RankMathUserSocialMeta interface vs making these fields directly available on RankMathUserSeo.

The latter would make the schema more robust to future changes (i.e. less likely for a change to necessitate something breaking), but I don't want a tech argument to win out if there's a practical use case I'm missing. 🤔

yep, that probably makes everything a bit easier 👍

[justlevine]

(Note to self: schema should resolve directly to the Object type not to the interface type. having to do ... on RankMathUserSeo when we're already at user.seo is stupid. Tracking in #79 )