Add Orbit trial state to the group GraphQL API

What does this MR do and why?

Adds the Orbit trial state to the group GraphQL API. The Orbit button uses it first.

• Group.orbitTrial (OrbitTrial, OrbitTrialState, OrbitTrialCredits).
• The orbit_trial feature flag and the read_orbit_trial permission for Owners.

CDot trial	Result
Can start a trial	ELIGIBLE
Cannot start a trial	NOT_ELIGIBLE
Active, with credits	ACTIVE
Expired, or 0 credits	ENDED
CustomersDot request fails	orbitTrial is null

The field resolves for one group per request. A later MR adds the start mutation.

References

gitlab-org/orbit#19

Closes gitlab-org/orbit/knowledge-graph#1334

How to set up and validate locally

With a local CustomersDot, in the Rails console:

Feature.enable(:orbit_trial, group)

Then, as an Owner of the group:

query {
  group(fullPath: "gitlab-org") {
    orbitTrial { state expiresOn credits { remaining total } }
  }
}

Demo against a local CustomersDot: eligible group, a Maintainer (no access), start (201), active, ended, and CDot down.

MR acceptance checklist

See the MR acceptance checklist.