Refactor snippet create and update service

What does this MR do?

In this MR we refactor a little bit the Snippets::CreateService and Snippets::UpdateService. There are common functionality and behaviors that we can refactor.

Besides, this refactor will be needed in future changes for implementing multiple files.

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