micro: rewrite options to support multiple building blocks

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

event.go

@@ -6,11 +6,22 @@ import (
 	"github.com/unistack-org/micro/v3/client"
 )
 
+// Event is used to publish messages to a topic
+type Event interface {
+	// Publish publishes a message to the event topic
+	Publish(ctx context.Context, msg interface{}, opts ...client.PublishOption) error
+}
+
 type event struct {
 	c     client.Client
 	topic string
 }
 
+// NewEvent creates a new event publisher
+func NewEvent(topic string, c client.Client) Event {
+	return &event{c, topic}
+}
+
 func (e *event) Publish(ctx context.Context, msg interface{}, opts ...client.PublishOption) error {
 	return e.c.Publish(ctx, e.c.NewMessage(e.topic, msg), opts...)
 }