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>

tests/functional/build-remote-with-mounted-ssh-ng.sh

@@ -5,17 +5,25 @@ source common.sh
 requireSandboxSupport
 [[ $busybox =~ busybox ]] || skipTest "no busybox"
 
+# An example of a command that uses the store only for its settings, to
+# make sure we catch needing the XP feature early.
+touch "$TEST_ROOT/foo"
+expectStderr 1 nix --store 'ssh-ng://localhost?mounted=%7B%7D' store add "$TEST_ROOT/foo" --dry-run | grepQuiet "experimental Nix feature 'mounted-ssh-store' is disabled"
+
 enableFeatures mounted-ssh-store
 
+# N.B. encoded query param is `mounted={}`. In the future, we can just
+# do `--store` with JSON, and then the nested structure will actually
+# bring benefits.
 nix build -Lvf simple.nix \
   --arg busybox "$busybox" \
   --out-link "$TEST_ROOT/result-from-remote" \
-  --store mounted-ssh-ng://localhost
+  --store 'ssh-ng://localhost?mounted=%7B%7D'
 
 nix build -Lvf simple.nix \
   --arg busybox "$busybox" \
   --out-link "$TEST_ROOT/result-from-remote-new-cli" \
-  --store 'mounted-ssh-ng://localhost?remote-program=nix daemon'
+  --store 'ssh-ng://localhost?mounted=%7B%7D&remote-program=nix daemon'
 
 # This verifies that the out link was actually created and valid. The ability
 # to create out links (permanent gc roots) is the distinguishing feature of