fetch metadata state of a borg repo (formats: text,json,...)

For monitoring purposes it would be nice to fetch the (full) ~~state~~ metadata state of a borg repo non-blocking, even if it is blocked. It could open the folder read only fetches all infos of the archives/blobs/... and drops the data structure as json

```bash
user@host:~$ borg state --full --json foorepo
{
...
}
user@host:~$ borg state --current-archive --json foorepo
{
...
}
```

Of this is achievable with overlayfs and list as well, but monitoring support of borg is very limited, so that it is quite hard to write general rules for systems like collectd, nagios, zabbix ...

Start by reviewing Borg's existing command-line and JSON-output behavior, then clarify the requested metadata scope and the meaning of non-blocking access when a repository is locked. Done should include an agreed command interface, supported output formats, and monitoring data for the requested repository states.

Written by the indexing model from the issue text.