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/legacy-ssh-store.hh

@@ -1,7 +1,7 @@
 #pragma once
 ///@file
 
-#include "ssh-store-config.hh"
+#include "common-ssh-store-config.hh"
 #include "store-api.hh"
 #include "ssh.hh"
 #include "callback.hh"
@@ -13,6 +13,11 @@ struct LegacySSHStoreConfig : virtual CommonSSHStoreConfig
 {
     using CommonSSHStoreConfig::CommonSSHStoreConfig;
 
+    LegacySSHStoreConfig(
+        std::string_view scheme,
+        std::string_view authority,
+        const Params & params);
+
     const Setting<Strings> remoteProgram{this, {"nix-store"}, "remote-program",
         "Path to the `nix-store` executable on the remote machine."};