Expose random sampling via api

[greenape]

I think with random sampling, while this is actually internally a distinct query class, it makes most sense to squash it into the query-to-be-sampled's params in the API, so it is always a modifier on another query.

e.g.

{
        "query_kind": "subscriber_degree",
        ...
        "sampling": {...}
    }

So, a schema that captures Random(..), and then post-load this is converted to the actual Random call.