Moved interfaces to own package

Description

The interfaces that did reside in nucleus have been moved to the newly created interfaces package.

Motivation and Context

This allows top level APIs like the PrincipalNetworkDomain type to have interfaces in its signature and avoids circular imports.

Types of changes

• 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:

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