feat(cli): add enterprise external-workspaces CLI command (#19287)

This pull request introduces support for external workspace management, allowing users to register and manage workspaces that are provisioned and managed outside of the Coder.

* coder external-workspaces create - Creates a new external workspace (this command extends coder create)
  * Example: coder external-workspaces create ext-workspace --template=externally-managed-workspace -y
  * Checks if template has coder_external_agent resource before creating a workspace
* coder external-workspaces list - Lists all external workspaces
* coder external-workspaces agent-instructions <workspace name> <agent name> - Retrieves agent connection instruction
  * Example: coder external-workspaces agent-instructions ext-workspace main --output=json

docs/reference/cli/external-workspaces_agent-instructions.md

@@ -0,0 +1,21 @@
+<!-- DO NOT EDIT | GENERATED CONTENT -->
+# external-workspaces agent-instructions
+
+Get the instructions for an external agent
+
+## Usage
+
+```console
+coder external-workspaces agent-instructions [flags] [user/]workspace[.agent]
+```
+
+## Options
+
+### -o, --output
+
+|         |                         |
+|---------|-------------------------|
+| Type    | <code>text\|json</code> |
+| Default | <code>text</code>       |
+
+Output format.