Normalize import_export structure

What does this MR do?

This brings a significant refactor to how we handle import_export.yml, merge it with EE and how we handle that for reader and saver.

This is meant to simplify the code, and remove a ton of conditions to handle different models of the structure.

This is also meant to prepare the structure to extend it much easier, like adding preload: or additional object types when needed.

This does not change the behavior of import/export, rather unifies and simplifies the current implementation.

This basically makes all internal handling to consume the structure that is:

tree:
  project:
    - labels:
      - :priorities
    - milestones:
      - events:
        - :push_event_payload

included_attributes:
  user:
    - :id

excluded_attributes:
  project:
    - :name

methods:
  labels:
    - :type
  events:
    - :action

preloads:
  statuses:
    project:

ee:
  tree:
    project:
      protected_branches:
        - :unprotect_access_levels
  included_attributes:
    user:
      - :name_ee
  excluded_attributes:
    project:
      - :name_without_ee
  methods:
    labels:
      - :type_ee
    events_ee:
      - :action_ee
  preloads:
    statuses:
      bridge_ee:

To be internally represented as:

{
  tree: {
    project: {
      labels: {
        priorities: {}
      },
      milestones: {
        events: {
          push_event_payload: {}
        }
      }
    }
  },
  included_attributes: {
    user: [:id]
  },
  excluded_attributes: {
    project: [:name]
  },
  methods: {
    labels: [:type],
    events: [:action]
  },
  preloads: {
    statuses: {
      project: nil
    }
  }
}

Previously the structure for tree: would be a mix of hashes, with arrays, with strings and symbols. This ensures that we always use uniform representation.

This is joint effort with @alipniagov.

This is slice of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32423.

In order to solve the https://gitlab.com/gitlab-org/gitlab-ce/issues/35389.

Does this MR meet the acceptance criteria?

Conformity

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