Port network/url command (#452)

* feat: add url command

* feat(network/url): add sub-command for url

crates/nu-protocol/src/signature.rs

@@ -42,6 +42,7 @@ pub enum Category {
     Filters,
     Formats,
     Math,
+    Network,
     Random,
     Platform,
     Shells,
@@ -64,6 +65,7 @@ impl std::fmt::Display for Category {
             Category::Filters => "filters",
             Category::Formats => "formats",
             Category::Math => "math",
+            Category::Network => "network",
             Category::Random => "random",
             Category::Platform => "platform",
             Category::Shells => "shells",