Allow Veterans to select multiple service-connected disabilities per secondary condition

Issue Description

per Shadowing research finding, Veterans often have multiple conditions contributing to another condition and expected to be able to indicate that

[ ] Desk research to understand Veteran-facing issue and scenarios under which it happens
[ ] Desk research and perhaps conversations to understand implications to the PDF and to VBA if we were to allow multiple conditions contributing to another condition
[ ] Design a way for Veterans to select multiple conditions contributing to another condition
[ ] (maybe) evaluative research (usability testing, etc) to assess the designed solution

[ ] Design completed
[ ] Design validated (by stakeholders and/or users)
[ ] Implementation ticket created

How to configure this issue
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[ ] Attached to an Epic (what body of work is this a part of?)
[ ] Labeled with Team (product support, analytics-insights, operations, service-design, Console-Services, tools-fe)
[ ] Labeled with Practice Area (backend, frontend, devops, design, research, product, ia, qa, analytics, contact center, research, accessibility, content)
[ ] Labeled with Type (bug, request, discovery, documentation, etc.)