Add the backend class, inputs, and request for inventory plugins.

[alexanderson1993]

Related Issue

Closes #261

How do you know the changes work correctly?

Added automated test

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.