feat: Establish workflow for lula validation identifiers

[brandtkeller]

Closes #226

Improving the validation identifiers to improve the developer experience and reduce potential errors due to miss identifying a link.

Changes

• Provide better separation between the OSCAL and Lula validations
  • BackMatterToMap() now strictly provides a map[string]string object for which identified validations will then be unmarshalled and placed in the validationMap.
    • This removes the need to place any identifier in the back-matter validation
    • This also provides a greater separation from OSCAL functionality and Lula Validation functionality
• Currently backwards compatible - this can be discussed and removed
• Establishes the use of rel for which we could expand to include validation "types"
• validationMap serves future external processing - such that when an external document is unmarshalled, we can place 1 -> N validations into the map rather than having to open the same file multiple times for different validations (reuse).
• Move the unmarshall and validation of a Validation from a String (back-matter.resource[_].description) to the common package to delineate that functionality from OSCAL functionality.

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