Expose all snippets to /snippets endpoint for GitLab Admin
### Problem to solve
Snippet spam is a problem for public GitLab instances, both self-managed and GitLab.com. Because of the spam issues with snippets Administrators are introducing additional tooling to help combat that spam. Unfortunately, there is not currently a way for a GitLab administrator to retrieve all Snippets on the system regardless of visibility or scope.
### Intended users
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
### Proposal
A new parameter should be available to the `/snippets` endpoint for GitLab Instance Administrators that allows them to retrieve all the Snippets on the instance.
Possible: `?scope=all`
### Testing Activity
@fjsanpedro:
* Add specs to ensure we include snippets with internal visibility in that endpoint
@vij:
* Test visible snippets are correct
### MR Breakdown
@fjsanpedro:
* We would need to change the scope we use for retrieving the public snippets and retrieve also internal (Maybe it would be better to include the SnippetFinder there?)
### Documentation
Docs need to be updated to specify that this parameter is Admin only and that it returns all snippets.
<details><summary>Original Description</summary>
### Problem to solve
Snippet spam is still a problem, including on https://gitlab.com. With the recent addition of the `/snippets/public` endpoint things have become much better: I wrote a script that deletes recent users that create accounts and immediately a snippet on our instance (same for users with unreasonably long bio written in Vietnamese). Unfortunately some spammers seem to also make snippets with internal visibility level.
### Intended users
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
### Proposal
I propose for the `/snippets/public` API endpoint to return all snippets visible to the current user, and not only snippets with public visibility. Alternatively I propose to introduce another endpoint that allows to list all snippets.
### Permissions and Security
No permissions necessary, and no potential for security problems.
</details>
issue