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.cc

@@ -1,5 +1,5 @@
 #include "legacy-ssh-store.hh"
-#include "ssh-store-config.hh"
+#include "common-ssh-store-config.hh"
 #include "archive.hh"
 #include "pool.hh"
 #include "remote-store.hh"
@@ -15,6 +15,15 @@
 
 namespace nix {
 
+LegacySSHStoreConfig::LegacySSHStoreConfig(
+    std::string_view scheme,
+    std::string_view authority,
+    const Params & params)
+    : StoreConfig(params)
+    , CommonSSHStoreConfig(scheme, authority, params)
+{
+}
+
 std::string LegacySSHStoreConfig::doc()
 {
     return
@@ -35,7 +44,7 @@ LegacySSHStore::LegacySSHStore(
     const Params & params)
     : StoreConfig(params)
     , CommonSSHStoreConfig(scheme, host, params)
-    , LegacySSHStoreConfig(params)
+    , LegacySSHStoreConfig(scheme, host, params)
     , Store(params)
     , connections(make_ref<Pool<Connection>>(
         std::max(1, (int) maxConnections),