Ensure we can construct remote store configs in isolation

Progress towards #10766

I thought that #10768 achieved, but when I went to use this stuff (in
Hydra), turns out it did not. (Those `using FooConfig;` lines were not
working --- they are so finicky!) This PR gets the job done, and adds
some trivial unit tests to make sure I did what I intended.

I had to add add a header to expose `SSHStoreConfig`, after which the
preexisting `ssh-store-config.*` were very confusingly named files, so I
renamed them to `common-ssh-store-config.hh` to match the type defined
therein.

src/libstore/ssh-store.hh

@@ -0,0 +1,28 @@
+#pragma once
+///@file
+
+#include "common-ssh-store-config.hh"
+#include "store-api.hh"
+#include "remote-store.hh"
+
+namespace nix {
+
+struct SSHStoreConfig : virtual RemoteStoreConfig, virtual CommonSSHStoreConfig
+{
+    using CommonSSHStoreConfig::CommonSSHStoreConfig;
+    using RemoteStoreConfig::RemoteStoreConfig;
+
+    SSHStoreConfig(std::string_view scheme, std::string_view authority, const Params & params);
+
+    const Setting<Strings> remoteProgram{
+        this, {"nix-daemon"}, "remote-program", "Path to the `nix-daemon` executable on the remote machine."};
+
+    const std::string name() override
+    {
+        return "Experimental SSH Store";
+    }
+
+    std::string doc() override;
+};
+
+}