Prepare config format & validation for beaconSets section

[amarthadan]

This section describes what beacons (array elements are beacon IDs) belong to a specific beacon set (the object keys are beacon set IDs):

"beaconSets": {
  "0x924b5d4cb3ec6366ae4302a1ca6aec035594ea3ea48a102d160b50b0c43ebfb5": [
    "0x924b5d4cb3ec6366ae4302a1ca6aec035594ea3ea48a102d160b50b0c43ebfb5",
    "0x924b5d4cb3ec6366ae4302a1ca6aec035594ea3ea48a102d160b50b0c43ebfb6"
  ]
},

In the validation, we need to make sure that every beacon listed has its counterpart in the beacons section.

[bbenligiray]

What if 0x924b5d4cb3ec6366ae4302a1ca6aec035594ea3ea48a102d160b50b0c43ebfb5 is also listed under beacons, would it be fetched twice, independently?
It feels like my answer to the first question here was wrong and we should always list all Beacons that are being updated (individually or as a set) under beacons.

[amarthadan]

Issue description updated based on the Slack discussion.