Feat(Backup): Add native google cloud storage backup support (#7829)

Currently, we use MinIO to write backup files to CGP storage. To do this we need to start a minio server (with GCP credentials) and trigger a backup with the destination as the minio server. This requires the user to run the minio server every time (or always) when a backup needs to be triggered. Using native SDK will help get rid of the extra service.

Google cloud storage (GCS) requires the user to create Service account key and credentials file. When starting an alpha we can set env variable:

GOOGLE_APPLICATION_CREDENTIALS="/path/to/credentials.json" dgraph alpha

to allow access to GCS. To trigger a backup the user can use the admin endpoint to initiate the backup. The destination should be set as:

"gs://<bucket_name>/path/to/backup"

go.mod

@@ -7,6 +7,7 @@ go 1.12
 // replace github.com/dgraph-io/roaring => /home/mrjn/go/src/github.com/dgraph-io/roaring
 
 require (
+	cloud.google.com/go/storage v1.15.0
 	contrib.go.opencensus.io/exporter/jaeger v0.1.0
 	contrib.go.opencensus.io/exporter/prometheus v0.1.0
 	github.com/Azure/azure-storage-blob-go v0.13.0
@@ -37,11 +38,12 @@ require (
 	github.com/gogo/protobuf v1.3.2
 	github.com/golang/geo v0.0.0-20170810003146-31fb0106dc4a
 	github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b
-	github.com/golang/protobuf v1.4.1
+	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
+	github.com/golang/protobuf v1.5.2
 	github.com/golang/snappy v0.0.2
 	github.com/google/codesearch v1.0.0
-	github.com/google/go-cmp v0.5.4
-	github.com/google/uuid v1.1.1
+	github.com/google/go-cmp v0.5.5
+	github.com/google/uuid v1.1.2
 	github.com/gorilla/websocket v1.4.2
 	github.com/graph-gophers/graphql-go v0.0.0-20200309224638-dae41bde9ef9
 	github.com/hashicorp/vault/api v1.0.4
@@ -65,15 +67,19 @@ require (
 	github.com/twpayne/go-geom v1.0.5
 	github.com/xdg/scram v0.0.0-20180814205039-7eeb5667e42c
 	go.etcd.io/etcd v0.0.0-20190228193606-a943ad0ee4c9
-	go.opencensus.io v0.22.5
+	go.opencensus.io v0.23.0
 	go.uber.org/zap v1.16.0
 	golang.org/x/crypto v0.0.0-20200820211705-5c72a883971a
-	golang.org/x/net v0.0.0-20201110031124-69a78807bb2b
-	golang.org/x/sync v0.0.0-20201020160332-67f06af15bc9
-	golang.org/x/sys v0.0.0-20210124154548-22da62e12c0c
-	golang.org/x/text v0.3.3
-	golang.org/x/tools v0.0.0-20210106214847-113979e3529a
-	google.golang.org/grpc v1.27.0
+	golang.org/x/lint v0.0.0-20210508222113-6edffad5e616 // indirect
+	golang.org/x/net v0.0.0-20210510120150-4163338589ed
+	golang.org/x/sync v0.0.0-20210220032951-036812b2e83c
+	golang.org/x/sys v0.0.0-20210511113859-b0526f3d8744
+	golang.org/x/text v0.3.6
+	golang.org/x/tools v0.1.1
+	google.golang.org/api v0.46.0
+	google.golang.org/genproto v0.0.0-20210510173355-fb37daa5cd7a // indirect
+	google.golang.org/grpc v1.37.1
+	google.golang.org/grpc/examples v0.0.0-20210518002758-2713b77e8526 // indirect
 	gopkg.in/DataDog/dd-trace-go.v1 v1.13.1 // indirect
 	gopkg.in/square/go-jose.v2 v2.3.1
 	gopkg.in/yaml.v2 v2.2.8