config/schema: support field deletion using :set()

This commit implements the `<schema object>:set()` algorithm in a more
accurate way and it solves several drawbacks of the previous
implementation.

* It was impossible to set a field that is nested to a record or a map
  that has the box.NULL value (#10190).
* It was impossible to set a field to the box.NULL value (#10193).
* It was impossible to delete a field, now `nil` RHS value means the
  deletion (#10194).

Fixes #10190
Fixes #10193
Fixes #10194

NO_DOC=Included into https://github.com/tarantool/doc/issues/4279