Add FileInput argument to snippet create mutation

What does this MR do?

This MR introduces the files param into the snippets create mutation. This param will store all the actions we want to perform over the snippet when we create it.

Ref #217795 (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