Improve saved configuration of the Aggregated Fields processor

Currently, the saved configuration of the "Aggregated Fields" processor looks something like this (example with one aggregated field defined):

      settings:
        fields:
          search_api_aggregation_1:
            label: 'Title &'
            type: union
            type_descriptions:
              union: ''
              concat: ''
              sum: ''
              count: ''
              max: ''
              min: ''
              first: ''
            fields:
              'entity:comment/subject': 'entity:comment/subject'
              'entity:node/title': 'entity:node/title'
              'entity:taxonomy_term/name': 'entity:taxonomy_term/name'
              'entity:user/name': 'entity:user/name'
              'entity:comment/changed': 0
              'entity:comment/field_name': 0
              'entity:comment/cid': 0
              'entity:comment/comment_type': 0
              'entity:comment/comment_body:processed': 0
              'entity:comment/comment_body': 0
              'entity:comment/comment_body:format': 0
              'entity:comment/created': 0
              'entity:comment/mail': 0
              'entity:comment/entity_id': 0
              'entity:comment/entity_type': 0
              'entity:comment/homepage': 0
              'entity:comment/hostname': 0
              'entity:comment/langcode': 0
              'entity:comment/name': 0
              'entity:comment/pid': 0
              'entity:comment/status': 0
              'entity:comment/thread': 0
              'entity:comment/uid': 0
              'entity:comment/uuid': 0
              'entity:node/uid': 0
              'entity:node/created': 0
              'entity:node/body': 0
              'entity:node/changed': 0
              'entity:node/field_image': 0
              'entity:node/langcode': 0
              'entity:node/nid': 0
              'entity:node/promote': 0
              'entity:node/status': 0
              'entity:node/vid': 0
              'entity:node/revision_log': 0
              'entity:node/revision_timestamp': 0
              'entity:node/revision_uid': 0
              'entity:node/sticky': 0
              'entity:node/field_tags': 0
              'entity:node/type': 0
              'entity:node/uuid': 0
              'entity:taxonomy_term/changed': 0
              'entity:taxonomy_term/description': 0
              'entity:taxonomy_term/langcode': 0
              'entity:taxonomy_term/tid': 0
              'entity:taxonomy_term/parent': 0
              'entity:taxonomy_term/uuid': 0
              'entity:taxonomy_term/vid': 0
              'entity:taxonomy_term/weight': 0
              'entity:user/changed': 0
              'entity:user/created': 0
              'entity:user/mail': 0
              'entity:user/init': 0
              'entity:user/langcode': 0
              'entity:user/access': 0
              'entity:user/login': 0
              'entity:user/pass': 0
              'entity:user/user_picture': 0
              'entity:user/preferred_admin_langcode': 0
              'entity:user/preferred_langcode': 0
              'entity:user/roles': 0
              'entity:user/signature': 0
              'entity:user/signature_format': 0
              'entity:user/timezone': 0
              'entity:user/uid': 0
              'entity:user/status': 0
              'entity:user/uuid': 0
            actions:
              remove: 'Remove field'
        actions:
          add: 'Add new Field'

Clearly, that needs to be significantly improved:

• For each aggregated field, only the label, type and fields keys are really needed.
• The fields key should just contain a (numerically indexed) sequence of the enabled fields, not the raw format used by the checkboxes form element. (That of course also means adapting the code that uses that configuration key, but that should be trivial enough.)

Estimated Value and Story Points

This issue was identified as a Beta Blocker for Drupal 8. We sat down and figured out the value proposition and amount of work (story points) for this issue.

Value and Story points are in the scale of fibonacci. Our minimum is 1, our maximum is 21. The higher, the more value or work a certain issue has.

Value : 1
Story Points: 3