feat(coderd): add support for external agents to API's and provisioner (#19286)

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.

Depends on: https://github.com/coder/terraform-provider-coder/pull/424

* GET /api/v2/init-script - Gets the agent initialization script
  * By default, it returns a script for Linux (amd64), but with query parameters (os and arch) you can get the init script for different platforms
* GET /api/v2/workspaces/{workspace}/external-agent/{agent}/credentials - Gets credentials for an external agent **(enterprise)**
* Updated queries to filter workspaces/templates by the has_external_agent field

cli/testdata/coder_list_--output_json.golden

@@ -70,7 +70,8 @@
         "most_recently_seen": null
       },
       "template_version_preset_id": null,
-      "has_ai_task": false
+      "has_ai_task": false,
+      "has_external_agent": false
     },
     "latest_app_status": null,
     "outdated": false,

cli/testdata/coder_provisioner_list_--output_json.golden

@@ -7,7 +7,7 @@
     "last_seen_at": "====[timestamp]=====",
     "name": "test-daemon",
     "version": "v0.0.0-devel",
-    "api_version": "1.8",
+    "api_version": "1.9",
     "provisioners": [
       "echo"
     ],