Support Multi-account widgets in newrelic_one_dashboard

[Efrat19]

Feature Description

We can improve the experience for clients using accounts partitioning, if we add a new field account_ids to nrql_query block to accept an array of account ids instead of the single string you can pass today as account_id.
This will be translated to a multi-account query, which is already supported on the nerdGraph API - widgets.rawConfiguration .nrqlQueries.[].accountIds and also on the currentnewrelic_one_dashboard.variable context see docs

Allow a nrql_queries field in the newrelic_one_dashboard.widget_* context. The new accept an array of nrql_query objects, will include 2 fields with the following types:

Support nrql_query.account_ids Field at the newrelic_one_dashboard.widget_*. Context
Currently in a json / manual dashboard implementation, a panel allows nrqlQueries

Describe Alternatives

Currently the alternatives are:

1. Use multiple nrql_query blocks in a widget block, each with its own account_id and the same query, as suggested here. This is a bad solution because instead of a combined result (for a multi-account query) you get multiple results (1 for each data account) you will see multliple graphs / billboard items etc. 🤮
2. Use newrelic_one_dashboard_raw and provide configuration. nrqlQueries[].accountIds to create the multi-account query. Unfortunately using newrelic_one_dashboard_raw isn't the preferred practice and isn't kept up to date, and also lacks features like adding variable blocks
3. Use newrelic_one_dashboard_json and provide configuration. nrqlQueries[].accountIds to create the multi-account query. This is possible but you have to sacrifice ease of using a human-maintainable, dynamic newrelic_one_dashboard.

Open questions

I started to work on a PR for this but realized the limit comes from the go-client pkg itself- is there a particular reason why nrqlQuery.AccountIDs is not supported here?