Add group contacts query to GraphQL

What does this MR do?

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

The tables/models are in place, and we just merged first GraphQL query to pull organizations for a group (!69318 (merged)).

This introduces the corresponding GraphQL query to pull contacts for a group.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Insert a few organizations for a group (flightjs perhaps):

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. Grab the inserted record id's:

select * from customer_relations_organizations;

3. Insert a few contacts for the group (some attached to organizations, some not):

insert into customer_relations_contacts (group_id, organization_id, created_at, updated_at, phone, first_name, last_name, email, description) values (26, 2, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, '07777 777777', 'Lee', 'Tickett', 'example@gitlab.com', 'VIP');
insert into customer_relations_contacts (group_id, organization_id, created_at, updated_at, phone, first_name, last_name, email, description) values (26, 2, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, null, 'Jane', 'Doe', null, null);
insert into customer_relations_contacts (group_id, organization_id, created_at, updated_at, phone, first_name, last_name, email, description) values (26, null, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, null, 'Marty', 'McFly', null, null);
insert into customer_relations_contacts (group_id, organization_id, created_at, updated_at, phone, first_name, last_name, email, description) values (26, null, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, null, 'Boy', 'George', null, null);

4. Execute GraphQL query:

query {
  group(fullPath: "flightjs") { 
    contacts {
      nodes {
        id
        organization {
          id
          name
          defaultRate
          description
          createdAt
          updatedAt
        }
        firstName
        lastName
        phone
        email
        description
        createdAt
        updatedAt
      }
    }
  }
}

Does this MR meet the acceptance criteria?

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Security

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Related to #2256