Add Google Cloud Storage as backend

Environment variables:
GOOGLE_PROJECT_ID=gcp-project-id
GOOGLE_APPLICATION_CREDENTIALS=path-to-json-file

Environment variables for test:
RESTIC_TEST_GS_PROJECT_ID=gcp-project-id
RESTIC_TEST_GS_APPLICATION_CREDENTIALS=path-to-json-file
RESTIC_TEST_GS_REPOSITORY=gs:us-central1/test-bucket

Init repository:
$ restic -r gs🪣/[prefix] init

internal/backend/gs/config.go

@@ -0,0 +1,57 @@
+package gs
+
+import (
+	"errors"
+	"path"
+	"strings"
+
+	"github.com/restic/restic/internal/options"
+)
+
+// Config contains all configuration necessary to connect to an gcs compatible
+// server.
+type Config struct {
+	ProjectID   string
+	JSONKeyPath string
+	Bucket      string
+	Prefix      string
+
+	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 20)"`
+}
+
+// NewConfig returns a new Config with the default values filled in.
+func NewConfig() Config {
+	return Config{
+		Connections: 5,
+	}
+}
+
+func init() {
+	options.Register("gs", Config{})
+}
+
+// ParseConfig parses the string s and extracts the gcs config. The
+// supported configuration format is gs:bucketName:/[prefix].
+func ParseConfig(s string) (interface{}, error) {
+	if strings.HasPrefix(s, "gs:") {
+		s = s[3:]
+	} else {
+		return nil, errors.New("gs: invalid format")
+	}
+	// use the first entry of the path as the bucket name and the
+	// remainder as prefix
+	path := strings.SplitN(s, ":/", 2)
+	return createConfig(path)
+}
+
+func createConfig(p []string) (interface{}, error) {
+	if len(p) < 2 {
+		return nil, errors.New("gs: invalid format, bucket name not found")
+	}
+	cfg := NewConfig()
+	cfg.Bucket = p[0]
+	if p[1] != "" {
+		cfg.Prefix = path.Clean(p[1])
+	}
+	return cfg, nil
+}