New multi-doc way to apply static configuration to network links.
Fixes#10957
Also implements related changes:
* new-style auto-DHCP mode - run DHCP as long as there is no new-style
config document
* refactoring of controllers/tests which were touched in this PR
* implemented embedded config structs handling in docgen -> Markdown and
docgen -> JSON schema
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>
7bae5b40b4