AggregationTrigger implementation

[rishabhmaurya]

• UTs for AggregationTrigger

Description of changes: Implementation of AggregationTrigger. It replaces and also introduces additional fields for AggregationTrigger. Trigger definition would look like -

  "triggers": [{
    "aggregation_trigger": {
      "name": "test-trigger",
      "severity": "1",
      "condition": {
        "buckets_path": {
          "avg": "goals_stats.avg"
        },
        "script": {
          "source": "params.avg >= 0 "
        },
        "parent_bucket_path": "<path_to_parent_bucket>",
        "filter": {
           "include": ["foo*", "bar*"]
        }
      },
      "actions": [{
         ...
      }]
    }
  }]

For details on usage of BucketSelectorExt pipeline aggregation, refer the documentation in PR: https://github.com/opendistro-for-elasticsearch/alerting/pull/374 By making a contribution to this project, I certify that:

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