Add new elasticsearch client (elastic#69905)

* add "@elastic/elasticsearch" to dependencies

* first POC of new client

* add logging

* add generation script for client facade API and implementation

* add back keepAlive

* add exports from client

* add new client mocks

* add some doc

* fix API usages

* rename legacy client to legacy in service

* rename currently unused config/client observable

* wire new client to service & update mocks

* fix mock type

* export client types

* add transport.request

* more doc

* migrate version_check to new client

* fix default port logic

* rename legacy client mocks

* move legacy client mocks to legacy folder

* start adding tests

* add configure_client tests

* add get_client_facade tests

* bump client to 7.8

* add cluster_client tests

* expose new client on internal contract only

* revert using the new client for es version check

* add service level test for new client

* update generated API

* Revert "rename legacy client mocks"

This reverts commit e48f3ad

* address some review comments

* revert ts-expect-error from unowned files

* move response mocks to mocks.ts

* Remove generated facade, use ES Client directly

* log queries even in case of error

* nits

* use direct properties instead of accessors

* handle async closing of client

* review nits

* ElasticSearchClient -> ElasticsearchClient

* add test for encoded querystring

* adapt test file
# Conflicts:
#	x-pack/test/upgrade_assistant_integration/upgrade_assistant/status.ts

package.json

@@ -122,6 +122,7 @@
     "@elastic/apm-rum": "^5.2.0",
     "@elastic/charts": "19.8.1",
     "@elastic/datemath": "5.0.3",
+    "@elastic/elasticsearch": "7.8.0",
     "@elastic/ems-client": "7.9.3",
     "@elastic/eui": "24.1.0",
     "@elastic/filesaver": "1.1.2",
@@ -289,7 +290,6 @@
   "devDependencies": {
     "@babel/parser": "^7.10.2",
     "@babel/types": "^7.10.2",
-    "@elastic/elasticsearch": "^7.4.0",
     "@elastic/eslint-config-kibana": "0.15.0",
     "@elastic/eslint-plugin-eui": "0.0.2",
     "@elastic/github-checks-reporter": "0.0.20b3",