Rclone storage backend (#4402)

* Add Rclone storage backend

* Support templating the name of files stored via Rclone

* Enable Rclone accounting

* Remove redundant type conversion

* Provide progress information for Rclone download/upload operations

* Log error when Rclone can't instantiate filesystem

* Remove filename templating functionality for Rclone storage

To (maybe) be later reintroduced as a generic functionality for all
storage backends.

* Remove S3 specific check

* Move Rclone config initialisation to init() method

weed/storage/backend/rclone_backend/rclone_progress.go

@@ -0,0 +1,19 @@
+package rclone_backend
+
+import "github.com/rclone/rclone/fs/accounting"
+
+type ProgressReader struct {
+	acc *accounting.Account
+	tr  *accounting.Transfer
+	fn  func(progressed int64, percentage float32) error
+}
+
+func (pr *ProgressReader) Read(p []byte) (n int, err error) {
+	n, err = pr.acc.Read(p)
+	if err != nil {
+		return
+	}
+	snap := pr.tr.Snapshot()
+	err = pr.fn(snap.Bytes, 100*float32(snap.Bytes)/float32(snap.Size))
+	return
+}