ESQL: API for listing running queries

[GalLalouche]

Create an API for listing currently running ESQL queries.

GET /_query/queries
// returns for each of the running queries
// query_id, start_time, running_time, query
// could add status one of {"running, cancelled, stopping"}
{ "queries" : {
 "abc": {
  "id": "abc",
  "start_time_millis": 14585858875292,
  "running_time_nanos": 762794,
  "query": "FROM logs* | STATS BY hostname"
  },
 "4321": {
  "id":"4321",
  "start_time_millis": 14585858823573,
  "running_time_nanos": 90231,
  "query": "FROM orders | LOOKUP country_code ON country"
  }
 } 
}

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)