Add new ExtendConnectivity functionality

[asaju7]

Proposed changes

Adds a new version of the ExtendConnectivity to work with Subcell in visualisations in SpECTRE. This PR adds most of the helpers that enable the new functionality. However, the current version of the function will only work for face neighbours (as defined in the connectivity DevGuide: #5825). Future versions will improve on this.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments