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

site/e2e/provisionerGenerated.ts

@@ -462,6 +462,7 @@ export interface PlanComplete {
    */
   hasAiTasks: boolean;
   aiTasks: AITask[];
+  hasExternalAgents: boolean;
 }
 
 /**
@@ -1395,6 +1396,9 @@ export const PlanComplete = {
     for (const v of message.aiTasks) {
       AITask.encode(v!, writer.uint32(114).fork()).ldelim();
     }
+    if (message.hasExternalAgents === true) {
+      writer.uint32(120).bool(message.hasExternalAgents);
+    }
     return writer;
   },
 };