feat(Login): Add a Flight Director core for seeing which clients are logged in.

[alexanderson1993]

Description

Creates a definition for a Login core which can be used as a template for building other cores.

Related Issue

Closes #274

How do you know the changes work correctly?

Just a code review - there isn't any new functionality yet.

Types of changes

• Docs Change (changes were only made to documentation)
• Refactor (non-breaking change which improves code)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  change)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the built-in project documentation.
• My change adds dependencies to this project.
• I have added tests to cover my changes.
• All new and existing tests passed.