Add support for external storages

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 [] TO [OPTIONS ]
```

`` should in the form of a function :
* `local({path=})` or `local()` or just path
* `ssh({host=, path=, username=, password=})`
* `s3({path=, key=, secret= })`
etc.

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

Start by reading the proposed SQL syntax and destination examples in the issue, then inspect the manticore-backup entry points for how backups currently choose their destination. The command-line syntax is explicitly unresolved, so clarify the scope and interface before implementation. Done means backup destinations can include local, SSH, and S3 storage, with the command-line behavior decided.

Written by the indexing model from the issue text.