The core uses per-domain folder structure

[mshustov]

Following #47065
The new platform enforces domain separation for the plugins. At the same time, all core services don't follow the per-domain folder structure.
It creates a situation when:

• core service code is scattered across several folders and there is no good way to share env-agnostic code and types. Migrate Capabilities to new platform #51438 (comment)
• lack of central place for service documentation.

This issue should evaluate how it's hard it would be to introduce proper per-domain separation and decide whether we want to follow this way at all.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

This will likely be addressed by #134112

[pgayvallet]

Addressed since the IDM initiative