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/initscript.go

@@ -0,0 +1,28 @@
+package codersdk
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+func (c *Client) InitScript(ctx context.Context, os, arch string) (string, error) {
+	url := fmt.Sprintf("/api/v2/init-script/%s/%s", os, arch)
+	res, err := c.Request(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return "", err
+	}
+	defer res.Body.Close()
+
+	if res.StatusCode != http.StatusOK {
+		return "", ReadBodyAsError(res)
+	}
+
+	script, err := io.ReadAll(res.Body)
+	if err != nil {
+		return "", err
+	}
+
+	return string(script), nil
+}