objectdb: add __rich_repr__

[skshetry]

This will format repr nicely if you use rich for printing (as I do in pdb and ipython).

This may be not useful for repr(db), but if you, for example have a very verbose output, this becomes very useful.

Eg:

StorageMapping(
    {
        (): StorageInfo(
            data=None,
            cache=ObjectStorage(
                key=(),
                odb=HashFileDB(
                    fs=<dvc_objects.fs.local.LocalFileSystem object at 0x1109e68d0>,
                    path='/private/var/folders/xh/trg29z296h70n109kwfk6g800000gn/T/pytest-of-saugat/pytest-278/pytest-servers13',
                    read_only=False
                ),
                index=None,
                read_only=False
            ),
            remote=None
        )
    }
)

StorageMapping({(): StorageInfo(data=None, cache=ObjectStorage(key=(), odb=HashFileDB(fs=<dvc_objects.fs.local.LocalFileSystem object at 0x1109e68d0>, path='/private/var/folders/xh/trg29z296h70n109kwfk6g800000gn/T/pytest-of-saugat/pytest-278/pytest-servers13', read_only=False), index=None, read_only=False), remote=None)})