Prepare ItemAction Factory for FHIR download

​🆕​ Feature

Create an ItemAction Factory and corresponding service that lets users download their data in FHIR format.

Affected app variants: Doc, Pat, Study

Ideas

This works analogously to the PDF export.

Tasks

Define specific tasks in the order they need to be done in. Reference files or specific lines of code that need to be changed.

• Create a service in features/src/fhir that converts patient data to FHIR format (use the core module)
• Add a ItemAction Factory to the fhir module that starts the download