Create GraphQL Type for PagesDomain

In order to move the pages UI to vue we need to expose Pages domains with GraphQL. We should add a GraphQL type named PagesDomain to the Project type. It can be queried on the Project as a simple list (instead of a connection) as I assume there will rarely be more than 10 domains per project.

The final type should follow the below spec:

type PagesDomain {
  id: ID!
  domain: String
  certificate: String
  verifiedAt: Datetime
  enabledUntil: Datetime
  removeAt: Datetime
  autoSslEnabled: Boolean
  certificateValidNotBefore: Datetime
  certificateValidNotAfter: Datetime
  certificateSource: CertificateSource
  wildcard: Boolean
  autoSslFailed: Boolean
}

enum CertificateSource {
  USER_PROVIDED
  GITLAB_PROVIDED
}

Implementation Guide

1. Familiarise yourself with our GraphQL API style guide (No need to fully read it, just be aware it's a handy reference)
2. Add the new type in a file at app/graphql/types/pages_domain.rb (You can use pages_deployment_type.rb as a reference
3. Add the type to app/graphql/types/project_type.rb (again, you can use pages_deployments as a reference)
4. For testing GraphQL, we prefer integration tests over unit tests. Add a spec to spec/requests/api/graphql/pages