To add support for server value increment

[AlbertGao]

Hi, I have a use case of updating a counter field in the realtime db, and i am looking for this ServerValue.Increment() API, seems not supported in golang sdk.

I want to update this field on the server side without having to read the original value and do not need to worry about multi-write conflicts.

- The doc for ServerValue.Increment():

https://firebase.google.com/docs/database/web/read-and-write#atomic_server-side_increments\\

- And a ref about the performance gain compare to transactional update:

https://www.faqcode4u.com/faq/59074/how-quickly-can-you-atomically-increment-a-value-on-the-firebase-realtime-database

- It's there in the node.js sdk seems

https://github.com/firebase/firebase-admin-node/blob/a510ef4599a5a8b9a968543f6479e2b6319c2923/src/database/database-namespace.ts#L106

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[Timesking]

After investigating the source code, you could just do this to make server value increment working.

type ServerValue struct {
	SV interface{} `json:".sv"`
}

type ServerValueIncrement struct {
	Increment int64 `json:"increment"`
}


...
...

	ref := client.NewRef(fmt.Sprintf("user_scores/%d/%s", 1, "score"))
	sv := ServerValue{
		SV: &ServerValueIncrement{
			Increment: 1,
		},
	}

	if err := ref.Set(context.TODO(), &sv); err != nil {
		log.Fatal(err)
	}

	fmt.Println("score increment successfully!")