Add raw snippet file endpoint

What does this MR do?

Adds a new API endpoint for returning the raw content of a given file at a given ref.

This will allow users to query for a specific file/ref combination, inline with the functionality added via the new raw snippet action in #217775 (closed)

Refs #218226 (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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

This MR can be manually tested using variations of the following:

curl --header "PRIVATE-TOKEN: <your_access_token>" "http://127.0.0.1:3000/api/v4/snippets/<snippet_id>/raw/<ref>/<snippet_file_name>"