[Discover] Implement data source context flavor resolution

[kertal]

🚧 Work in progress. Placeholder issue for implementing the flavor resolution process 🚧

Today Discover supports three different data sources:

• ES|QL query
• Data view (persisted and ad hoc)
• Saved search (contains an ES|QL query or data view reference)

For One Discover, this list will need to extend to an unknown number of data sources, which the current URL structure and application state are not well suited for. To support this requirement, we will introduce an abstraction called a “data source” which represents some target for data exploration in Discover. An example of a new data source type would be “integration”.

type DiscoverDataSource =
  | { type: 'esql'; query: string }
  | { type: 'dataView'; dataViewId: string }
  | { type: 'savedSearch'; savedSearchId: string }
  | { type: 'integration'; integrationIds: string[] };
``

During the context resolution process, the data source will be used to fetch associated metadata, such as indices and fields, and then normalized to an internal structure Discover knows how to handle.

Additionally, the resolution process will return a data source context, used to make decisions about which code paths to follow in the flavor layer. 

[elasticmachine]

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

[kertal]

Closing, is part of #181962