Add GraphQL snippet FileInputType

What does this MR do?

This MR introduces the new GraphQL input type FileInputType that will be used to create/update snippet files in the repository through GraphQL mutations.

Refs #217789 (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