[Discover] Add support for default histogram breakdown fields and custom breakdown colour mappings

[stratoula]

📓 Summary

For the Discover logs data source profile, we want to set the default histogram breakdown field to log.level (and do similar for other profiles in the future). Additionally, we want to assign specific colour mappings and ordering to the breakdown values to better represent the data to users:

There are two separate tasks needed to support this in the contextual awareness framework:

• The getDefaultAppState extension point should be updated to allow defining a breakdown field as part of the default profile state.
• A new extension point should be added to Discover to support defining custom breakdown colour mappings, e.g. getBreakdownColorMappings, including custom ordering. The extension point should be separate from the default profile state because it should possible for a single profile to define colour mappings for multiple fields, not just the default breakdown field. The custom ordering portion of this work is blocked by custom breakdown ordering in Lens: [Lens] Custom Breakdown Presentation Order #191988.

Currently a PoC exists demonstrating how this could work for ES|QL mode, but the log.level field is hard coded and does not use an extension point: #190213. It can be used as a reference for how colour mappings can be passed to the Lens embeddable.

✔️ Acceptance criteria

• Update the getDefaultAppState extension point to support defining a default histogram breakdown field.
• Add a new extension point in Discover to support defining custom breakdown colour mappings, including custom sorting. The extension point should allow a single profile to define custom colour mappings for multiple fields.
• Ensure the changes work in both ES|QL mode and data view mode.
• Document the extension point changes, and add example profile implementations as well as functional tests for the new functionality.

[elasticmachine]

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

[stratoula]

I didnt add the blocked label because there is an open PR already and we can def start working on it without this feature.

[kertal]

elastic/elastic-charts#2524 has been merged 🥳