[Security Solutions][Detection Engine] Throws date time format errors on custom mappings

[FrankHassanabad]

Kibana version:
7.9.0+

Describe the bug:
When you have a custom timestamp mapping and run the detection engine against that mapping it can produce errors or it can not detect signals depending on what the timestamp happens to be.

Solution is to use format everywhere in ranges:
https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-range-query.html

Such as here:
https://github.com/elastic/kibana/blob/master/x-pack/plugins/security_solution/server/lib/detection_engine/signals/build_events_query.ts#L40-L46

Likewise docValues for when we pull the @timestamp out:
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-request-docvalue-fields.html

Steps to reproduce:
Add a epoch mapping and then get a valid epoch from here:
https://www.epochconverter.com/

PUT /frank-test-delme
{
  "mappings": {
    "dynamic": "strict",
    "properties": {
        "@timestamp": {
            "type": "date",
            "format": "epoch_millis"
        }
    }
  }
}

PUT frank-test-delme/_doc/1
{
  "@timestamp": 1602085855941
}

Then create a detection rule against the index and you will see:

Errors:

erver    log   [09:52:52.147] [error][plugins][plugins][securitySolution][securitySolution] Bulk Indexing of signals failed: [parse_exception] failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]: [failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]] :: {"path":"/frank-test-delme/_search","query":{"allow_no_indices":true,"size":100,"ignore_unavailable":true},"body":"{\"query\":{\"bool\":{\"filter\":[{\"bool\":{\"must\":[],\"filter\":[{\"match_all\":{}}],\"should\":[],\"must_not\":[]}},{\"bool\":{\"filter\":[{\"bool\":{\"should\":[{\"range\":{\"@timestamp\":{\"gte\":\"2020-10-07T15:46:51.254Z\"}}}],\"minimum_should_match\":1}},{\"bool\":{\"should\":[{\"range\":{\"@timestamp\":{\"lte\":\"2020-10-07T15:52:51.254Z\"}}}],\"minimum_should_match\":1}}]}},{\"match_all\":{}}]}},\"sort\":[{\"@timestamp\":{\"order\":\"asc\"}}]}","statusCode":400,"response":"{\"error\":{\"root_cause\":[{\"type\":\"parse_exception\",\"reason\":\"failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]: [failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]]\"}],\"type\":\"search_phase_execution_exception\",\"reason\":\"all shards failed\",\"phase\":\"query\",\"grouped\":true,\"failed_shards\":[{\"shard\":0,\"index\":\"frank-test-delme\",\"node\":\"UmFIfD_iQfepBl6sUyEk2g\",\"reason\":{\"type\":\"parse_exception\",\"reason\":\"failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]: [failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]]\",\"caused_by\":{\"type\":\"illegal_argument_exception\",\"reason\":\"failed to parse date field [2020-10-07T15:46:51.254Z] with format [epoch_millis]\",\"caused_by\":{\"type\":\"date_time_parse_exception\",\"reason\":\"date_time_parse_exception: Failed to parse with all enclosed parsers\"}}}}]},\"status\":400}"} name: "host create after" id: "02cbbb9f-4759-4fcd-a615-adf6bbfd1019" rule id: "b4cf64d9-ce06-45a5-98ce-2f2b1354c830" signals index: ".siem-signals-hassanabad4-default"

Expected behavior:
No errors, the rule as expected if the custom mapping is Epoch