Add fields to GraphQL snippet blob type

What does this MR do?

In this MR we add the fields external_storage and rendered_as_text to the GraphQL snippet blob type.

Refs #217434 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content