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

codersdk/workspacebuilds.go

@@ -90,6 +90,7 @@ type WorkspaceBuild struct {
 	TemplateVersionPresetID *uuid.UUID           `json:"template_version_preset_id" format:"uuid"`
 	HasAITask               *bool                `json:"has_ai_task,omitempty"`
 	AITaskSidebarAppID      *uuid.UUID           `json:"ai_task_sidebar_app_id,omitempty" format:"uuid"`
+	HasExternalAgent        *bool                `json:"has_external_agent,omitempty"`
 }
 
 // WorkspaceResource describes resources used to create a workspace, for instance: