Update lowdown version

Fix #4042

According to https://github.com/kristapsdz/lowdown/commit/8aef9e9290de22a10c14ae138257bc1c7fa8ba1f, we shouldn't need to use a fork anymore so we can switch back to upstream

Fix the nix-daemon Mac OS SSL CA cert

nix-prefetch-url: Add --executable flag

pkgs.fetchurl supports an executable argument, which is especially nice
when downloading a large executable. This patch adds the same option to
nix-prefetch-url.

I have tested this to work on the simple case of prefetching a little
executable:

1. nix-prefetch-url --executable https://my/little/script
2. Paste the hash into a pkgs.fetchurl-based package, script-pkg.nix
3. Delete the output from the store to avoid any misidentified artifacts
4. Realise the package script-pkg.nix
5. Run the executable

I repeated the above while using --name, as well.

I suspect --executable would have no meaningful effect if combined with
--unpack, but I have not tried it.

find() returns an iterator so "!attr" doesn't work.

Remove storetype delegate reg store -- contains #3736

Fix garbage collection of CA derivations

Remove corepkgs/config.nix

Fix #4026

This isn't used anywhere except in the configure script of the Perl
bindings. I've changed the latter to use the C++ API's Settings object
at runtime.

Mac OS multi-user installations are currently broken because all requests
made by nix-daemon to the binary cache fail with:

```
unable to download ... Problem with the SSL CA cert (path? access rights?) (77).
```

This change ensures that the nix-daemon knows where to find the SSL CA cert file.

Fixes #2899 and #3261.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

Doesn't test much, but at least ensures that the command runs properly

Add some necessary casts in the initialisation of the store's config

Instead make a separate header with the template implementation of
`BaseSetting<T>::toJSONObj` that can be included where needed

It is apparently required for using `toJSONObject()`, which we do inside
the header file (because it's in a template).

This was accidentally working when building Nix itself (presumably because
`config.hh` was always included after `nlohman/json.hpp`) but caused a
(pretty dirty) build failure in the perl bindings package.

Work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80431 that was
already there in the code but was accidentally removed in the last
commits

So that it can be printed by `nix describe-stores`

Allow `-` and `.` in the RFC schemes as stated by
[RFC3986](https://tools.ietf.org/html/rfc3986#section-3.1).

Practically, this is needed so that `ssh-ng` is a valid URI scheme

When opening a store, only try the stores whose `uriSchemes()` include
the current one

Using virtual inheritance means that only the default constructors of
the parent classes will be called, which isn't what we want

Using the `*Config` class hierarchy

Rework the `Store` hierarchy so that there's now one hierarchy for the
store configs and one for the implementations (where each implementation
extends the corresponding config). So a class hierarchy like

```
StoreConfig-------->Store
    |                 |
    v                 v
SubStoreConfig----->SubStore
    |                 |
    v                 v
SubSubStoreConfig-->SubSubStore
```

(with virtual inheritance to prevent DDD).

The advantage of this architecture is that we can now introspect the configuration of a store without having to instantiate the store itself

The default value is initialized when creating the setting and unchanged
after that