feat(manager): Add a manager to support unified driver switching

[flc1125]

For Example:

package main

import (
	"context"

	"github.com/ThreeDotsLabs/watermill"
	"github.com/ThreeDotsLabs/watermill/manager"
	"github.com/ThreeDotsLabs/watermill/message"
	"github.com/ThreeDotsLabs/watermill/pubsub/gochannel"
)

func main() {
	m := manager.New(gochannel.NewGoChannel(
		gochannel.Config{},
		watermill.NewStdLogger(false, false),
	))

	m.Register("another", gochannel.NewGoChannel(
		gochannel.Config{},
		watermill.NewStdLogger(false, false),
	))
	// or other pubsub drivers, e.g.: nas/kafka...

	m.Publish("topic", message.NewMessage(watermill.NewUUID(), []byte("Hello, world!")))
	m.Use().Publish("topic", message.NewMessage(watermill.NewUUID(), []byte("Hello, world!")))
	m.Use("another").Publish("topic", message.NewMessage(watermill.NewUUID(), []byte("Hello, world!")))

	m.Subscribe(context.Background(), "topic")
	m.Use().Subscribe(context.Background(), "topic")
	m.Use("another").Subscribe(context.Background(), "topic")
}

Originally, I wanted to put it under the https://github.com/ThreeDotsLabs/watermill package, but there is a circular dependency in the current architecture. So, I created a manager package.

[m110]

Thanks for contributing @flc1125!

The code looks good to me, although we decided to avoid adding more APIs for working with the Pub/Subs to keep it straightforward and explicit. It's short enough that someone could keep it in their project if they want. Alternatively, it could be a good candidate for a separate repository. :)