feat(cli): add open app <workspace> <app-slug> command (#17032)

Fixes https://github.com/coder/coder/issues/17009

Adds a CLI command `coder open app <workspace> <app-slug>` that allows
opening arbitrary `coder_apps` via the CLI.

Users can optionally specify a region for workspace
applications.

cli/testdata/coder_open_--help.golden

@@ -6,6 +6,7 @@ USAGE:
   Open a workspace
 
 SUBCOMMANDS:
+    app       Open a workspace application.
     vscode    Open a workspace in VS Code Desktop
 
 ———

cli/testdata/coder_open_app_--help.golden

@@ -0,0 +1,14 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder open app [flags] <workspace> <app slug>
+
+  Open a workspace application.
+
+OPTIONS:
+      --region string, $CODER_OPEN_APP_REGION (default: primary)
+          Region to use when opening the app. By default, the app will be opened
+          using the main Coder deployment (a.k.a. "primary").
+
+———
+Run `coder --help` for a list of global options.