Validate before conversion

[aj-stein-nist]

User Story:

As a developer or engineer using oscal-cli, in order to ensure that invalid (but well-formed) OSCAL content is not converted between raw data formats, I would like to be able to optionally validate before conversion, and warn if converting invalid content even if well-formed.

Background: While completing integration tests to smoke test all subcommands for usnistgov/oscal-cli#178, I determined that in some cases the CLI will convert between XML and JSON and YAML when data is well-formed (valid JSON or XML or YAML), even if it does not conform with the schema and Metaschema-constraint requirements for an instance of a given model.

Goals:

• [ ] Make validation before conversion the default processing approach (I am open to warning or halting conversion)
• [ ] Implement a command-line flag to enforce validation before conversion (I would like to be able to disable this for performance reasons for very large OSCAL document instances)

Dependencies:

N/A

Acceptance Criteria

• [ ] All website and readme documentation affected by the changes in this issue have been updated.
• [ ] A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• [ ] The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.