GSP-837: Support Feature Flag

[JinnyYi]

ref: #835

[Xuanwo]

In GSP-87: Feature Gates, we have the concept of features first. And then in GSP-109: Redesign Features, we have a huge redesign about it.

We have the following features for now:

• LoosePair
• VirtualDir
• VirtualObjectMetadata

It's obvious that they are only a subset of all features we support.

In fact, we can categorize all features into three kinds:

• Native support
• No native support but could be virtual
• Can't support

In the past, we only care about features that "No native support but could be virtual". Because other features can be handled easily: if we support, everything will work fine; If not, we just don't implement the interface.

However, GSP-751: Write Empty File Behavior rise a new question: How to handle the different behavior of the same interface?

Maybe it's time for us to drop the idea that we introduced in GSP-1: Unify storager behavior.

Maybe we can merge all interface together into Storager and than implement:

type Storage interface {    
	Feature() Feature
}

if store.Feature().CanCopy() {
    err := x.Copy(oldpath, newpath)
    if err != nil {
        return err
    }
}

[Xuanwo]

Mostly LGTM, let's create a tracking issue for this GSP and call for the final review.

[Xuanwo]

Bravo work！