(feat): ElasticRepository for Feed Impressions

See #468 (closed) for CQL schema

{
  "properties": {
    "uuid": {
      "type": "keyword"
    },
    "entity_urn": {
      "type": "keyword"
    },
    "entity_type": {
      "type": "keyword"
    },
    "entity_guid": {
      "type": "long"
    },
    "source": {
      "type": "keyword"
    },
    "campaign": {
      "type": "keyword"
    },
    "medium": {
      "type": "keyword"
    },
    "position": {
      "type": "integer"
    },
    "@timestamp": {
      "type": "date"
    }
  }
}