azurefiles: fix missing x-ms-file-request-intent header

According to the SDK docs

> FileRequestIntent is required when using TokenCredential for
> authentication. Acceptable value is backup.

This sets the correct option in the SDK. It does it for all types of
authentication but the SDK seems clever enough not to supply it when
it isn't needed.

This fixes the error

> MissingRequiredHeader An HTTP header that's mandatory for this
> request is not specified. x-ms-file-request-intent

Fixes #8241

backend/azurefiles/azurefiles.go

@@ -393,8 +393,10 @@ func newFsFromOptions(ctx context.Context, name, root string, opt *Options) (fs.
 	policyClientOptions := policy.ClientOptions{
 		Transport: newTransporter(ctx),
 	}
+	backup := service.ShareTokenIntentBackup
 	clientOpt := service.ClientOptions{
-		ClientOptions: policyClientOptions,
+		ClientOptions:     policyClientOptions,
+		FileRequestIntent: &backup,
 	}
 
 	// Here we auth by setting one of cred, sharedKeyCred or f.client