azurefiles: finish docs and implementation and add optional interfaces

- use rclone's http Transport
- fix handling of 0 length files
- combine into one file and remove uneeded abstraction
- make `chunk_size` and `upload_concurrency` settable
- make auth the same as azureblob
- set the Features correctly
- implement `--azurefiles-max-stream-size`
- remove arbitrary sleep on Mkdir
- implement `--header-upload`
- implement read and write MimeType for objects
- implement optional methods
    - About
    - Copy
    - DirMove
    - Move
    - OpenWriterAt
    - PutStream
- finish documentation
- disable build on plan9 and js

Fixes #365
Fixes #7378

backend/azurefiles/azurefiles_test.go

@@ -1,3 +1,6 @@
+//go:build !plan9 && !js
+// +build !plan9,!js
+
 package azurefiles
 
 import (