Update trestle project structure to better define the SDK APIs

[jpower432]

Issue description / feature objectives

To support the trestle SDK user, create well-defined APIs so users can understand the SDK functionality what API signatures cannot be changed without breaking backward compatibility.

Caveats / Assumptions

• Keep package as one published package per discussion on https://github.com/oscal-compass/community/pull/68

Completion Criteria

• [ ] Define the common utilities that are used in both the SDK and CLI (e.g. list-utils)
• [ ] Define logic that supports the CLI command functionality only and what supports the trestle SDK
• [ ] Split the SDK into its own sub-package of trestle

Related to #924