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/common-ssh-store-config.cc

@@ -0,0 +1,43 @@
+#include <regex>
+
+#include "common-ssh-store-config.hh"
+#include "ssh.hh"
+
+namespace nix {
+
+static std::string extractConnStr(std::string_view scheme, std::string_view _connStr)
+{
+    if (_connStr.empty())
+        throw UsageError("`%s` store requires a valid SSH host as the authority part in Store URI", scheme);
+
+    std::string connStr{_connStr};
+
+    std::smatch result;
+    static std::regex v6AddrRegex("^((.*)@)?\\[(.*)\\]$");
+
+    if (std::regex_match(connStr, result, v6AddrRegex)) {
+        connStr = result[1].matched ? result.str(1) + result.str(3) : result.str(3);
+    }
+
+    return connStr;
+}
+
+CommonSSHStoreConfig::CommonSSHStoreConfig(std::string_view scheme, std::string_view host, const Params & params)
+    : StoreConfig(params)
+    , host(extractConnStr(scheme, host))
+{
+}
+
+SSHMaster CommonSSHStoreConfig::createSSHMaster(bool useMaster, Descriptor logFD)
+{
+    return {
+        host,
+        sshKey.get(),
+        sshPublicHostKey.get(),
+        useMaster,
+        compress,
+        logFD,
+    };
+}
+
+}