Add FileInput argument to update mutation

What does this MR do?

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

Refs #217799 (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