Add workItemsStateCount field to namespace type

What does this MR do and why?

This MR adds the workItemsStateCount field to the namespace GQL type, as a follow up to #537460 (closed). The aim is to allow the frontend to request the state counts without needing to differentiate whether the request should be made under group.workItemStateCounts or project.workItemStateCounts.

References

• #550033 (closed)

Screenshots or screen recordings

Not provided

How to set up and validate locally

1. In an environment with the GDK running, navigate to http://localhost:3000/-/graphql-explorer
2. Write a query which fetches workItemStateCounts for groups, projects, and namespaces simultaneously. To simplify testing, I also pulled the fullPath out into a variable. i.e:

Query:

query q($fullPath: ID!) {
  namespace(fullPath: $fullPath) {
    workItemStateCounts { all closed opened}
  }
  group(fullPath: $fullPath) {
    workItemStateCounts { all closed opened}
  }
  project(fullPath: $fullPath) {
    workItemStateCounts { all closed opened}
  }
}

Variables:

{ "fullPath": "flightjs/flight" }

3. Run the query, updating the fullPath variable to the path of a group, subgroup, project and user namespace. You should observe:

• When called on a group, the counts for group.workItemStateCounts should match namespace.workItemStateCounts
• When called on a subgroup, the counts for group.workItemStateCounts should match namespace.workItemStateCounts
• When called on a project, the counts for project.workItemStateCounts should match namespace.workItemStateCounts
• When called on a user namespace, null should be returned for all of the counts (unsupported)

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Related to #550033 (closed)