New store settings system

Motivation:

See the linked issues for details.

The most notable user-relevant bits are:

- This cleans up the `MountedSSHStore`: decomposed into its orthogonal parts

- This brings us pretty close to being able to then implement a JSON-based config.
   - Store query parameters can be JSON
   - Stores can entirely be specified via JSON objects, but this is not yet hooked up to anything.

Also behind the scenes have these benefits:

1. The docs are moved out of the headers, good for less rebuilding when they changes
2. Stores are always constructed from store configs
3. Use JSON, avoid custom serializers

Context:

Part of #11106

Co-Authored-By: Robert Hensing <robert@roberthensing.nl>
Co-authored-by: Sergei Zimmerman <145775305+xokdvium@users.noreply.github.com>

src/libstore/local-binary-cache-store.cc

@@ -8,12 +8,21 @@
 
 namespace nix {
 
+config::SettingDescriptionMap LocalBinaryCacheStoreConfig::descriptions()
+{
+    config::SettingDescriptionMap ret;
+    ret.merge(StoreConfig::descriptions());
+    ret.merge(BinaryCacheStoreConfig::descriptions());
+    return ret;
+}
+
+
 LocalBinaryCacheStoreConfig::LocalBinaryCacheStoreConfig(
     std::string_view scheme,
     PathView binaryCacheDir,
     const StoreReference::Params & params)
     : Store::Config{params}
-    , BinaryCacheStoreConfig{params}
+    , BinaryCacheStoreConfig{*this, params}
     , binaryCacheDir(binaryCacheDir)
 {
 }
@@ -32,9 +41,9 @@ struct LocalBinaryCacheStore :
 {
     using Config = LocalBinaryCacheStoreConfig;
 
-    ref<Config> config;
+    ref<const Config> config;
 
-    LocalBinaryCacheStore(ref<Config> config)
+    LocalBinaryCacheStore(ref<const Config> config)
         : Store{*config}
         , BinaryCacheStore{*config}
         , config{config}
@@ -126,10 +135,7 @@ StringSet LocalBinaryCacheStoreConfig::uriSchemes()
 }
 
 ref<Store> LocalBinaryCacheStoreConfig::openStore() const {
-    return make_ref<LocalBinaryCacheStore>(ref{
-        // FIXME we shouldn't actually need a mutable config
-        std::const_pointer_cast<LocalBinaryCacheStore::Config>(shared_from_this())
-    });
+    return make_ref<LocalBinaryCacheStore>(ref{shared_from_this()});
 }
 
 static RegisterStoreImplementation<LocalBinaryCacheStore::Config> regLocalBinaryCacheStore;