Decision task checks for all missing scopes before task submission

[ahal]

Imported from: https://bugzilla.mozilla.org/show_bug.cgi?id=1416858

Aki writes:
Currently, the decision task builds the task graph in memory in several steps, then submits the tasks to the queue. The first task submission to fail with scope errors will kill the decision task with an error message. However, when adding a number of new tasks and scopes, it's possible that the missing scopes listed in the error message are only a subset of the scopes needed for submitting the complete graph.

It would be great to have the decision task calculate all the scopes required for the task graph, and determine if it has sufficient scopes, before submitting any tasks. Then when we file scopes bugs, we'd be able to request the full set of missing scopes, rather than the currently failing subset.

[ahal]

So I put up a WIP PR that partially solves this:
#103

But the problem is that it only verifies the scopes that tasks explicitly set in their definitions. As is the verification does not handle any implicit scopes (e.g, for things like routes or the create-task scopes for the queue).

The verification could try to "guess" the implicit scopes that will be needed, which should likely be pretty stable and work fairly well. Though I don't love that the set of guessed scopes could diverge / be lacking from the set of real scopes that are needed. Ideally there would be an official API endpoint or utility function provided by Taskcluster that could return the full set of scopes needed given a task definition.