Import group members during group migration

From #298745 (closed)

Problem to solve

In order to associate imported group epics (and any other group relation) with their original authors we need to import group members into the group first.

Proposed solution

Create a new pipeline that runs prior to any other relation pipeline in order to map users between instances and create group members for found users. If user is not found - do nothing (user creation is out of scope of this issue).

A GraphQL query can look something like this:

query groupMembers($full_path: ID!, $cursor: String) {
  group(fullPath: $full_path) {
    groupMembers(relations: DIRECT, first: 100, after: $cursor) {
      page_info: pageInfo {
        end_cursor: endCursor
        has_next_page: hasNextPage
      }
      nodes {
        created_at: createdAt
        updated_at: updatedAt
        expires_at: expiresAt
        access_level: accessLevel {
          integer_value: integerValue
        }
        user {
          public_email: publicEmail
        }
      }
    }
  }
}

1. Create new pipeline to import group members (e.g. GroupMemberPipeline) with the query above
2. Fetch direct group members using GraphQL with the query above
3. For fetched user, if publicEmail is present, find by primary/secondary email such user. If it's found - create groupMember with this user. If not - do nothing.
4. If publicEmail is not present - do nothing.