feat: expose `refreshToken.authTokenExpiration`

[justlevine]

What

This PR adds the authTokenExpiration field to the refreshToken mutation payload.

Why

API oversight, flagged by @richardaubin in Slack

How

Testing Instructions

mutation refreshToken(
  $token: String! # The user's refreshToken.
) {
  refreshToken( input: {refreshToken: $token} ) {
    authToken
    authTokenExpiration # 👈
    success
  }
}

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.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

coverage: 82.619% (+0.05%) from 82.571% when pulling acb3972335ba703c5ff60096b9d87415313d0e72 on justlevine:feat/refreshToken-authTokenExpiration into a4e0771e1554ca232209de3f5297efb8cb09d249 on AxeWP:develop.

[richardaubin]

tried it... works fine.