[Discover] Support query languages

[stratoula]

Describe the feature:
Exactly as described here, we would like to support text-based languages in discover. At the moment, a user can select a dataview/saved search and interact with the fields list.

We want to enhance the dataview picker (part of the unified search project) to give the users the ability to select a text-based language. The search bar is going to be replaced by a code editor (part of the unified_search component) and some other parts of the UI are going to be hidden as they might not make sense.

Discover will trigger a search via the “SearchSource” interface in case of KQL mode and use the search service with the sql strategy in case SQL mode. The data fetching will be part of the discover app while the unified search plugin will hold the current state.

Open questions

• Do we want to integrate them with filter pills / time range picker? (or replace with SQL box?)
• Design finalization

[elasticmachine]

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

[kertal]

Linking #134654 for visibility of @elastic/kibana-data-discovery efforts

[stratoula]

I am closing this as this PR #136702 was merged in main and it contains the main functionality for supporting text-based languages in Discover. I have created a new issue for the follow-ups.#136950