Support ForEach queries

[sordina]

This isn't required for simple N+1 relationships, we would use this for optimised commands.

For example, a function definition could support batch api calls if exposed with array parameter annotation.

/**
 * Answers the question: `Are these users active?`
 * @pure
 * @param ids user-ids to query
 * @batch user_is_active ids id
 */
function users_are_active(ids: Array<string>): boolean {
    ...
}

Exposes users_are_active as normal, and user_is_active as if it had id parameter instead of ids and uses Variables capability to wire up relationships.

N+1 Engine Feature: https://hasurahq.atlassian.net/browse/V3API-194

While we design a solution that would allow per-function batch behaviour declaration that the engine can dispatch on, we could implement blankey ForEach support in TS connector and make the decision to perform a batch call in the connector.

[paf31]

One problem right now is that arguments are just Values, so we would need a way to specify them via variables. But the idea seems good in general to me.