Make GraphQL timeout configurable

From https://gitlab.com/gitlab-org/security/gitlab/merge_requests/35#note_264440903:

I think it would be nice if we can configurable in omnibus instead of just as an ENV variable.

This is fine for now though since this is a security MR.

cc @mdelaossa @johnhope

I'm not sure if this is a good idea, but I created this issue to discuss. We can make this public after #31225 (closed) is made public.