Add user@address:port support

This patch allows users to specify the connection port
in the store URLS like so:

```
nix store info --store "ssh-ng://localhost:22" --json
```

Previously this failed with: `error: failed to start SSH connection to 'localhost:22'`,
because the code did not distinguish the port from the hostname. This
patch remedies that problem by introducing a ParsedURL::Authority type
for working with parsed authority components of URIs.

Now that the URL parsing code is less ad-hoc we can
add more long-awaited fixes for specifying SSH connection
ports in store URIs.

Builds upon the work from bd1d2d1041.

Co-authored-by: Sergei Zimmerman <sergei@zimmerman.foo>
Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libfetchers/git-lfs-fetch.cc

@@ -44,16 +44,19 @@ static void downloadToSink(
 
 static std::string getLfsApiToken(const ParsedURL & url)
 {
+    assert(url.authority.has_value());
+
+    // FIXME: Not entirely correct.
     auto [status, output] = runProgram(
         RunOptions{
             .program = "ssh",
-            .args = {*url.authority, "git-lfs-authenticate", url.path, "download"},
+            .args = {url.authority->to_string(), "git-lfs-authenticate", url.path, "download"},
         });
 
     if (output.empty())
         throw Error(
             "git-lfs-authenticate: no output (cmd: ssh %s git-lfs-authenticate %s download)",
-            url.authority.value_or(""),
+            url.authority.value_or(ParsedURL::Authority{}).to_string(),
             url.path);
 
     auto queryResp = nlohmann::json::parse(output);