Support subscriptions in GraphQL explorer

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/158333



Merged-by: Alexandru Croitor <acroitor@gitlab.com>
Approved-by: Nwanna Isong <nisong@gitlab.com>
Approved-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Luke Duncalfe <lduncalfe@gitlab.com>
Approved-by: Alexandru Croitor <acroitor@gitlab.com>
Reviewed-by: Lukas Eipert <leipert@gitlab.com>
Reviewed-by: Heinrich Lee Yu <heinrich@gitlab.com>
Co-authored-by: Heinrich Lee Yu <heinrich@gitlab.com>
Co-authored-by: Lukas Eipert <leipert@gitlab.com>