R: add WithBinding in fetch

cloudflare/fetch/client.go

@@ -12,9 +12,30 @@ type Client struct {
 	namespace js.Value
 }
 
-// NewClient returns new Client
-func NewClient() *Client {
-	return &Client{
-		namespace: jsutil.Global,
+// applyOptions applies client options.
+func (c *Client) applyOptions(opts []ClientOption) {
+	for _, opt := range opts {
+		opt(c)
 	}
 }
+
+// ClientOption is a type that represents an optional function.
+type ClientOption func(*Client)
+
+// WithBinding changes the objects that Fetch API belongs to.
+// This is useful for service bindings, mTLS, etc.
+func WithBinding(bind js.Value) ClientOption {
+	return func(c *Client) {
+		c.namespace = bind
+	}
+}
+
+// NewClient returns new Client
+func NewClient(opts ...ClientOption) *Client {
+	c := &Client{
+		namespace: jsutil.Global,
+	}
+	c.applyOptions(opts)
+
+	return c
+}