Support ES client version upgrade to v8.0

[mshustov]

You can find the full Elasticsearch client roadmap here elastic/elasticsearch-js#1542
There are a few things that may require additional work from the Core side:

• Remove the body key from the request. effort for Core: Small, providing the client gives a long migration period for the legacy style, otherwise it affects nearly all the usages in Kibana codebase.
• The returned value of API calls is the body and not the HTTP related keys. effort for Core: Small, providing client allows Kibana to specify { meta: true } on the client level and ships kibana.d.ts with ApiResponse-comptible typings.
  @elastic/elasticsearch client continue shipping a Kibana-specific d.ts to support 7.x-compatible interfaces by default. After 8.0 we can consider an option to allow plugins to specify meta:false to return body as response.
• Migrate to the "undici" http client. We need to investigate what incompatibilities Kibana might have with the configuration parameters of the new http agent. The ES client supports configuring what http client should be used. To be on the safe side, we can continue using the native http agent. Migration to the undici agent can be done in a follow-up PR.
  

  kibana/src/core/server/elasticsearch/client/client_config.ts

  Lines 22 to 38 in 3950c43

  	export type ElasticsearchClientConfig = Pick<
  	ElasticsearchConfig,
  	| 'customHeaders'
  	| 'sniffOnStart'
  	| 'sniffOnConnectionFault'
  	| 'requestHeadersWhitelist'
  	| 'sniffInterval'
  	| 'hosts'
  	| 'username'
  	| 'password'
  	| 'serviceAccountToken'
  	> & {
  	pingTimeout?: ElasticsearchConfig['pingTimeout'] | ClientOptions['pingTimeout'];
  	requestTimeout?: ElasticsearchConfig['requestTimeout'] | ClientOptions['requestTimeout'];
  	ssl?: Partial<ElasticsearchConfig['ssl']>;
  	keepAlive?: boolean;
  	};

• Rename ssl option to tls effort for Core: Small, we aren't going to rename elasticsearch.ssl config, just handle the renaming internally.
• Move from emitter-like interface to a diagnostic method effort for Core: Small

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)