[Feature] GQL mutation to set gallery cover

[Valkyr-JS]

It's not currently possible to set a custom gallery cover image from inside Stash. The issue has been raised previously on Discord as well as here (https://github.com/stashapp/stash/issues/1432, https://github.com/stashapp/stash/issues/872).

I looked at creating a plugin to do this, but the blocker is that setting the cover image isn't currently possible in a GQL mutation. If the relevant function was exposed, then a plugin could be built to achieve this, at least until it becomes a native feature.