feat: add support for mediaDetails and featured images to Cover/MediaText

[justlevine]

What

This PR:

• exposes the mediaDetails field to the CoreCover and CoreMediaText objects.
• fixes the connectedMediaItem when handling CoreCover or CoreMediaText blocks that useFeaturedImage.

Why

Shim until we can fix this for real in WPGraphQL Content Blocks

Related Issue(s):

Part of https://github.com/rtCamp/headless/issues/228

How

Important

Only mediaDetails and connectedMediaItem support Featured Images.
Hydrating the Core{Cover|MediaText}Attributes fields can only be done in WPGraphQL Content Blocks. (After #25).

Testing Instructions

query GetTemplate($uri: String!) {
  templateByUri(uri: $uri) {
    editorBlocks {
      ... on CoreMediaText { # or ... on CoreCover
        attributes {
          useFeaturedImage
          ...MyAttributesFrag
        }
        mediaDetails {
         ...MyMediaDetailsFrag
        }
        connectedMediaItem {
          node {
            ...MyConnectedMediaItemNodeFrag
          }
        }
      }
    }
  }
}


## Screenshots
<!-- Include relevant screenshots proving the PR works as indended. -->

## Additional Info
<!-- Please include any relevant logs, error output, etc -->

Unit tests have _not_ been included, since this is a temporary shim. The functionality is smoke-tested in the frontend block components.

## Checklist
<!-- We encourage you to complete this checklist to the best of your abilities. If you can't do everything, that's okay too. -->
- [x] I have read the [Contribution Guidelines](../DEVELOPMENT.md).
- [x] My code is tested to the best of my abilities.
- [x] My code passes all lints (PHPCS, PHPStan, ESLint, etc). <!-- See the Contributing Guidelines for linting instructions -->
- [x] My code has detailed inline documentation. <!-- Guidelines: https://developer.wordpress.org/coding-standards/inline-documentation-standards/php/ -->
- [ ] I have added unit tests to verify the code works as intended.
- [x] I have updated the project documentation accordingly.

[justlevine]

connectedMediaItem fixed: