Add Azure Blob Storage as backend

Environment variables:
AZURE_ACCOUNT_NAME=storage-account-name
AZURE_ACCOUNT_KEY=storage-account-key

Environment variables for test:
RESTIC_TEST_AZURE_ACCOUNT_NAME=storage-account-name
RESTIC_TEST_AZURE_ACCOUNT_KEY=storage-account-key
RESTIC_TEST_AZURE_REPOSITORY=azure:restic-test-container

Init repository:
$ restic -r azure:container-name:/prefix/dir init

internal/backend/azure/azure_test.go

@@ -0,0 +1,121 @@
+package azure_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/restic/restic/internal/backend/azure"
+	"github.com/restic/restic/internal/backend/test"
+	"github.com/restic/restic/internal/restic"
+	. "github.com/restic/restic/internal/test"
+)
+
+func newAzureTestSuite(t testing.TB) *test.Suite {
+	return &test.Suite{
+		// do not use excessive data
+		MinimalData: true,
+
+		// NewConfig returns a config for a new temporary backend that will be used in tests.
+		NewConfig: func() (interface{}, error) {
+			azcfg, err := azure.ParseConfig(os.Getenv("RESTIC_TEST_AZURE_REPOSITORY"))
+			if err != nil {
+				return nil, err
+			}
+
+			cfg := azcfg.(azure.Config)
+			cfg.AccountName = os.Getenv("RESTIC_TEST_AZURE_ACCOUNT_NAME")
+			cfg.AccountKey = os.Getenv("RESTIC_TEST_AZURE_ACCOUNT_KEY")
+			cfg.Prefix = fmt.Sprintf("test-%d", time.Now().UnixNano())
+			return cfg, nil
+		},
+
+		// CreateFn is a function that creates a temporary repository for the tests.
+		Create: func(config interface{}) (restic.Backend, error) {
+			cfg := config.(azure.Config)
+
+			be, err := azure.Create(cfg)
+			if err != nil {
+				return nil, err
+			}
+
+			exists, err := be.Test(context.TODO(), restic.Handle{Type: restic.ConfigFile})
+			if err != nil {
+				return nil, err
+			}
+
+			if exists {
+				return nil, errors.New("config already exists")
+			}
+
+			return be, nil
+		},
+
+		// OpenFn is a function that opens a previously created temporary repository.
+		Open: func(config interface{}) (restic.Backend, error) {
+			cfg := config.(azure.Config)
+			return azure.Open(cfg)
+		},
+
+		// CleanupFn removes data created during the tests.
+		Cleanup: func(config interface{}) error {
+			cfg := config.(azure.Config)
+
+			be, err := azure.Open(cfg)
+			if err != nil {
+				return err
+			}
+
+			if err := be.(restic.Deleter).Delete(context.TODO()); err != nil {
+				return err
+			}
+
+			return nil
+		},
+	}
+}
+
+func TestBackendAzure(t *testing.T) {
+	defer func() {
+		if t.Skipped() {
+			SkipDisallowed(t, "restic/backend/azure.TestBackendAzure")
+		}
+	}()
+
+	vars := []string{
+		"RESTIC_TEST_AZURE_ACCOUNT_NAME",
+		"RESTIC_TEST_AZURE_ACCOUNT_KEY",
+		"RESTIC_TEST_AZURE_REPOSITORY",
+	}
+
+	for _, v := range vars {
+		if os.Getenv(v) == "" {
+			t.Skipf("environment variable %v not set", v)
+			return
+		}
+	}
+
+	t.Logf("run tests")
+	newAzureTestSuite(t).RunTests(t)
+}
+
+func BenchmarkBackendAzure(t *testing.B) {
+	vars := []string{
+		"RESTIC_TEST_AZURE_ACCOUNT_NAME",
+		"RESTIC_TEST_AZURE_ACCOUNT_KEY",
+		"RESTIC_TEST_AZURE_REPOSITORY",
+	}
+
+	for _, v := range vars {
+		if os.Getenv(v) == "" {
+			t.Skipf("environment variable %v not set", v)
+			return
+		}
+	}
+
+	t.Logf("run tests")
+	newAzureTestSuite(t).RunBenchmarks(t)
+}