Add option to pass a pair of file descriptors as alternative to ``--rsh``

## Have you checked borgbackup docs, FAQ, and open Github issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

Feature request, I guess?

## System information. For client/server mode post info for both machines.

#### Your borg version (borg -V).

borg 1.1.10

#### Operating system (distribution) and version.

Debian GNU/Linux bullseye/sid

#### Hardware / network configuration, and filesystems used.

N/A

#### How much data is handled by borg?

N/A

## Describe the problem you're observing.

I am writing a few things to document and ease pull-style operation (#900). During this, I came across the issue that I somehow need to pass a socket or a pair of file descriptors to borg instead of a rsh command.

Of course, I can emulate that using a shell script like this as RSH:

```bash
#!/bin/bash
exec socat "UNIX-CONNECT:$BORGWRAP_SOCKET_PATH" STDIO
```

However, it would be much easier if we could simply pass file descriptor numbers to ``--rsh`` on platforms which support that. Alternatively, pass a path to a unix socket to connect to.