Add `uploaded_files` param to Snippet create mutation

What does this MR do?

This MR adds the argument uploaded_files to the snippet create mutation. This argument is needed to send which files were uploaded to the snippet description. That same info is what the actual controllers send.

Refs #217932 (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