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

coderd/workspacebuilds.go

@@ -1157,6 +1157,11 @@ func (api *API) convertWorkspaceBuild(
 		aiTasksSidebarAppID = &build.AITaskSidebarAppID.UUID
 	}
 
+	var hasExternalAgent *bool
+	if build.HasExternalAgent.Valid {
+		hasExternalAgent = &build.HasExternalAgent.Bool
+	}
+
 	apiJob := convertProvisionerJob(job)
 	transition := codersdk.WorkspaceTransition(build.Transition)
 	return codersdk.WorkspaceBuild{
@@ -1185,6 +1190,7 @@ func (api *API) convertWorkspaceBuild(
 		TemplateVersionPresetID: presetID,
 		HasAITask:               hasAITask,
 		AITaskSidebarAppID:      aiTasksSidebarAppID,
+		HasExternalAgent:        hasExternalAgent,
 	}, nil
 }