[EQLSearchStrategy] Clean up and align with other async strategies

[Dosant]

Similar like we did to low level SQL strategy, we should clean up EQL search strategy and align it with other async strategies:

Ideally we should do it before wider adoption in Kibana (unified search).

Here is the list of possible alignment things:

• Is it Required to return both headers + body in the response?

  kibana/src/plugins/data/common/search/strategies/eql_search/types.ts

  Line 22 in 74fc0fb

  export type EqlSearchStrategyResponse<T = unknown> = IKibanaSearchResponse<TransportResult<T>>;

  . Can we do body + warning as in other strategies?
• Can we better type it using EqlSearchResponse ?
• Don't use deprecatated typesWithBodyKey format
• Type search request with EqlSearchRequest and EqlGetRequest
• Don't provide unsupported by strategies params / options to avoid ts-ignore:

  kibana/src/plugins/data/server/search/strategies/eql_search/eql_search_strategy.ts

  Line 60 in 74fc0fb

  : // @ts-expect-error optional key cannot be used since search doesn't expect undefined

  . Make params explicit
• Properly use submit / get async utils
• Add example plugin
• Add function test for example plugin
• Add API test for search strategy

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesSv)

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)