Add group organizations query to GraphQL

What does this MR do?

This is part of the ongoing effort to add the concept of organizations and contacts (see #2256).

The tables/models are in place. This introduces the first GraphQL query to pull organizations for a group.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Create a few organizations for a group (flightjs for example)

gdk psql
insert into customer_relations_organizations (group_id, created_at, updated_at, default_rate, name, description) values (26, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, 123.45, 'Tickett', 'Monthly retainer £1,000');
insert into customer_relations_organizations (group_id, created_at, updated_at, default_rate, name, description) values (26, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, 1000, 'Tickett', 'No monthly retainer');

2. Execute graphql query...

query {
  group(fullPath: "flightjs") { 
    name
    id
    organizations {
      nodes {
        id
        name
        defaultRate
        description
        createdAt
        updatedAt
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #2256