Wishlist: support PAX and BORG `--tar-format` in borg 1.4.x

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

Yes

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

Feature request

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

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

1.2.2

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

Linux (Ubuntu 22.04, Manjaro, Armbian - debian)

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

Not relevant

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

Not relevant

#### Full borg commandline that lead to the problem (leave away excludes and passwords)

Not relevant

## Describe the problem you're observing.

Would like to condense here some discussion that happened in other issues/threads. Now that `--tar-format` has been merged in `master` with support for `PAX` and `BORG` formats (the last supporting lossless roundtrip), it would be great if a point release of borg 1.2.x could be made with these features.

Rationale:

- Borg 1.2.x is currently completely missing a lossless or way to move archives from repo to repo. Having `tar-format=BORG` could provide such support, even if in not in the efficient way planned for borg2.
- Even if borg2 is in the working, it will probably still take time for it to come and then it will not likely immediately supersede borg 1.2.x in adoption and in some cases borg 1.2.x will be kept around for some time. Providing it with the ability to perform lossless export/import would be useful, and I hope not to expensive, being the feature already developed.
- At this point there are probably other fixes accumulated, so that a borg 1.2.3 is probably planned anyway.