Serialization of markdown style images references into a oscal model.

[butler54]

Issue description / feature objectives

Referencing: https://github.com/GSA/fedramp-automation/blob/master/documents/Guide_to_OSCAL-based_FedRAMP_System_Security_Plans_(SSP).pdf

and:

Image data can be included in documents in two ways:

1. Explicitly by diagram element in an SSP
2. Implicitly by an html image (in XML) or markdown (in json / yaml)

In order to have a deliverable that is transferable as one document, images would need to be serialised into base64 encoding and attached to the back-matter.

Expected Behavior

• string elements are scanned for markdown image tags as well as diagram elements.
• Any external image references are resolved, the field is updated, and the image is serialized.

Remarks

• The open question here is 'when' is this resolution performed (e.g. during ssp assembly or during the distribution / release phase), the later would be more effective as users may edit links at other stages.

[butler54]

Based on use within online platforms support can be limited to gif, jpg and png.