[Investigate]: refactor types to use pointers similar to the oscal created types and lula native types

[mike-winberry]

Describe what should be investigated or refactored

I have noticed a lot of frustrations when consuming go-oscal as a library both within itself and within Lula. Many of the issues I have run into would likely be solved by not passing around instantiated "empty" structs, by replacing many of the structs with pointers, it opens it up to simple nil checks and also ensures json compatibility with go-oscal Library types and consistency with Lula and this projects own generated types

Links to any relevant code

• extraneous check due to default (empty) struct:

  go-oscal/src/pkg/validation/validator.go

  Lines 98 to 103 in 004addd

  	func (v *Validator) GetValidationResult() (ValidationResult, error) {
  	if v.validationResult.TimeStamp == (ValidationResult{}).TimeStamp {
  	return v.validationResult, errors.New("validation has not been run")
  	}
  	return v.validationResult, nil
  	}

Additional context

catalyst for creating this issue:

• will solve
• wont solve created: #ISSUE_NUMBER

Add any other context or screenshots about the technical debt here.