feat(schema): basic storage settings (#1455)

Extend JSON schema to partially support the new *storage config*
described in this document:
https://github.com/openSUSE/agama/blob/master/doc/auto_storage.md.

This PR only extends the schema for the most basic settings (i.e.,
*boot*, *drives* and *partitions*). Settings for advanced searches,
delete devices, creeate RAIDs, etc are not considered yet.

Notes:

* The *search* section added in this PR is based on the following
proposal:
https://gist.github.com/joseivanlopez/4f2381eb1f6c3aca562aadc2ad920a72?permalink_comment_id=5121860#gistcomment-5121860.
This will be revisited later.
* The *guided* schema added by
#1263 is not going to be offered
(e.g., *$ agama config load* will not understand it). The *storage
config* will cover all the use cases and features provided by the
*guided config*.
* The *guided config* will be used only by clients (e.g., web UI) to
easily configure a subset of the use cases supported by the *storage
config*.