New Kibana header design

[ryankeairns]

Relates to #62010
Relates to #58049

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As part of the navigation redesign project, a new stacked header design will be implemented that sets us up for global navigation across Elastic products. This issue covers Phase 1 of the effort; phases are described in #62010

Design proposal

Proposed changes

👉 Pre-requisite: New/updated EuiHeader component(s) (add link to EUI issue/PR once availble)

1. A new, second (upper/black) header bar that will contain:
   • Home button (links to Cloud eventually?)
   • Deployment switcher (later)
   • Global search (separate issue Global Search MVP #58049)
   • Support
   • Profile
2. The current (lower/white) header bar will contain:
   • Navigation menu toggle button
   • Spaces button
   • Breadcrumb navigation (will evolve to include Cloud in future)
   • Application menu (relocation to be handled by app teams and/or Kibana Design)

Rollout plan

Phase 1

Implement the first version of the new stacked header

• Swap in the new/updated components from EUI
• Relocate global search (presuming it begins in the existing, single-row header)
• Relocate app menus into the header

The first version will include the aforementioned changes with the following caveats:

• Global search will not yet search across deployments. This project will be likely still be in its MVP state (returning applications and saved objects for the current Space of the current instance), with backend work progressing to support the ability to register additional search content. That latter effort has not been fully discussed/planned (i.e. needs cross-team involvement), but the registry approach would essentially allow plugin authors to register additional data sources thus also enabling a true cross-deployment search.
• In addition to the scope of search, there will also be a concerted effort to move application menu bars into the lower header. The EUI team has handled such migrations before and is prepared to guide/assist solution teams in this effort. This change will ultimately be at their own discretion and, by opting out, they can simply keep their existing app menu. The advantage of relocating their app menu is that they gain back vertical real estate.

[elasticmachine]

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