azure: add support for service principals - fixes #3230

Before: users can only connect to Azure blob containers using the access keys
from the storage account.

After: users can additionally choose connect to Azure blob containers
using service principals. This uses OAuth2 under the hood to exchange
a client ID and client secret for a short-lived access token.

Ref:
- https://github.com/rclone/rclone/issues/3230
- https://docs.microsoft.com/en-us/azure/storage/common/storage-auth-aad-app?tabs=dotnet#well-known-values-for-authentication-with-azure-ad
- https://docs.microsoft.com/en-us/azure/developer/go/azure-sdk-authorization#available-authentication-types-and-methods
- https://gist.github.com/ItalyPaleAle/ec6498bfa81a96f9ca27a2da6f60a770

backend/azureblob/azureblob_test.go

@@ -5,10 +5,12 @@
 package azureblob
 
 import (
+	"context"
 	"testing"
 
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fstest/fstests"
+	"github.com/stretchr/testify/assert"
 )
 
 // TestIntegration runs integration tests against the remote
@@ -35,3 +37,33 @@ var (
 	_ fstests.SetUploadChunkSizer = (*Fs)(nil)
 	_ fstests.SetUploadCutoffer   = (*Fs)(nil)
 )
+
+// TestServicePrincipalFileSuccess checks that, given a proper JSON file, we can create a token.
+func TestServicePrincipalFileSuccess(t *testing.T) {
+	ctx := context.TODO()
+	credentials := `
+{
+    "appId": "my application (client) ID",
+    "password": "my secret",
+    "tenant": "my active directory tenant ID"
+}
+`
+	tokenRefresher, err := newServicePrincipalTokenRefresher(ctx, []byte(credentials))
+	if assert.NoError(t, err) {
+		assert.NotNil(t, tokenRefresher)
+	}
+}
+
+// TestServicePrincipalFileFailure checks that, given a JSON file with a missing secret, it returns an error.
+func TestServicePrincipalFileFailure(t *testing.T) {
+	ctx := context.TODO()
+	credentials := `
+{
+    "appId": "my application (client) ID",
+    "tenant": "my active directory tenant ID"
+}
+`
+	_, err := newServicePrincipalTokenRefresher(ctx, []byte(credentials))
+	assert.Error(t, err)
+	assert.EqualError(t, err, "error creating service principal token: parameter 'secret' cannot be empty")
+}