* explore: try to use existing mode when switching datasource

* elasticsearch: initial explore logs support

* Elasticsearch: Adds ElasticsearchOptions type
Updates tests accordingly

* Elasticsearch: Adds typing to query method

* Elasticsearch: Makes maxConcurrentShardRequests optional

* Explore: Allows empty query for elasticsearch datasource

* Elasticsearch: Unifies ElasticsearchQuery interface definition
Removes check for context === 'explore'

* Elasticsearch: Removes context property from ElasticsearchQuery interface
Adds field property
Removes metricAggs property
Adds typing to metrics property

* Elasticsearch: Runs default 'empty' query when 'clear all' button is pressed

* Elasticsearch: Removes index property from ElasticsearchOptions interface

* Elasticsearch: Removes commented code from ElasticsearchQueryField.tsx

* Elasticsearch: Adds comment warning usage of for...in to elastic_response.ts

* Elasticsearch: adds tests related to log queries