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/libstore/store-reference.cc

@@ -48,7 +48,7 @@ StoreReference StoreReference::parse(const std::string & uri, const StoreReferen
         auto parsedUri = parseURL(uri);
         params.insert(parsedUri.query.begin(), parsedUri.query.end());
 
-        auto baseURI = parsedUri.authority.value_or("") + parsedUri.path;
+        auto baseURI = parsedUri.authority.value_or(ParsedURL::Authority{}).to_string() + parsedUri.path;
 
         return {
             .variant =