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.

tests/unit/libstore/legacy-ssh-store.cc

@@ -0,0 +1,26 @@
+#include <gtest/gtest.h>
+
+#include "legacy-ssh-store.hh"
+
+namespace nix {
+
+TEST(LegacySSHStore, constructConfig)
+{
+    LegacySSHStoreConfig config{
+        "ssh",
+        "localhost",
+        StoreConfig::Params{
+            {
+                "remote-program",
+                // TODO #11106, no more split on space
+                "foo bar",
+            },
+        }};
+    EXPECT_EQ(
+        config.remoteProgram.get(),
+        (Strings{
+            "foo",
+            "bar",
+        }));
+}
+}