Add support for external storages

[sanikolaev]

It will makes things easier if we support backing up directly to an external storage like ssh and s3.

Here's what the syntax may look like in SQL:

BACKUP [<what>] TO <where> [OPTIONS <options>]  

<where> should in the form of a function :

• local({path=}) or local(<path>) or just path
• ssh({host=, path=, username=, password=})
• s3({path=, key=, secret= })
  etc.

The command line syntax for manticore-backup is to be discussed.