Use SnapshotSource for generating SD Snapshots

[EniacMlezi]

Description

The snapshot generation done by the Validator does not automatically generate for structure definitions which again refer to other structure definitions. This is a known issue with a known solution.

Related issues

Addresses #4018.

Testing

Tested by running the failing case in #4018. Needs some more testing for different use cases

FHIR Team Checklist

• Update the title of the PR to be succinct and less than 65 characters
• Add a milestone to the PR for the sprint that it is merged (i.e. add S47)
• Tag the PR with the type of update: Bug, Build, Dependencies, Enhancement, New-Feature or Documentation
• Tag the PR with Open source, Azure API for FHIR (CosmosDB or common code) or Azure Healthcare APIs (SQL or common code) to specify where this change is intended to be released.
• Tag the PR with Schema Version backward compatible or Schema Version backward incompatible or Schema Version unchanged if this adds or updates Sql script which is/is not backward compatible with the code.
• CI is green before merge
• Review squash-merge requirements

Semver Change (docs)

Patch|Skip|Feature|Breaking (reason)

[brendankowitz]

/azp run

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