config: add a new `concretizer` config section

The concretizer is going to grow to have many more configuration,
and we really need some structured config for that.

* We have the `config:concretizer` option that chooses the solver,
  but extending that is awkward (we'd need to replace a string with
  a `dict`) and the solver choice will be deprecated eventually.

* We have the `concretization` option in environments, but it's
  not a top-level config section -- it's just for environments,
  and it also only admits a string right now.

To avoid overlapping with either of these and to allow the most
extensibility in the future, this adds a new `concretizer` config
section that can be used in and outside of environments. There
is only one option right now: `reuse`.  This can expand to include
other options later.

Likely, we will soon deprecate `config:concretizer` and warn when
the user doesn't use `clingo`, and we will eventually (sometime later)
move the `together` / `separately` options from `concretization` into
the top-level `concretizer` section.

This commit just adds the new section and schema. Fully wiring it
up is TBD.

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