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 rclone#365
Fixes rclone#7378

README.md

@@ -59,6 +59,7 @@ Rclone *("rsync for cloud storage")* is a command-line program to sync files and
   * Mega [:page_facing_up:](https://rclone.org/mega/)
   * Memory [:page_facing_up:](https://rclone.org/memory/)
   * Microsoft Azure Blob Storage [:page_facing_up:](https://rclone.org/azureblob/)
+  * Microsoft Azure Files Storage [:page_facing_up:](https://rclone.org/azurefiles/)
   * Microsoft OneDrive [:page_facing_up:](https://rclone.org/onedrive/)
   * Minio [:page_facing_up:](https://rclone.org/s3/#minio)
   * Nextcloud [:page_facing_up:](https://rclone.org/webdav/#nextcloud)