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/coderd.go

@@ -1566,6 +1566,9 @@ func New(options *Options) *API {
 			r.Use(apiKeyMiddleware)
 			r.Get("/", api.tailnetRPCConn)
 		})
+		r.Route("/init-script", func(r chi.Router) {
+			r.Get("/{os}/{arch}", api.initScript)
+		})
 	})
 
 	if options.SwaggerEndpoint {