borg2: implement tag command

[ThomasWaldmann]

As borg2 is now using all archive metadata directly from the archive object, having some more metadata in there would be useful.

CLI interface could look like this:

borg tag aid:<archive_id> --add TAG1 --add TAG2
borg tag aid:<archive_id> --remove TAG1 --remove TAG2
borg tag aid:<archive_id> --set TAG1 --set TAG2  # replace all existing tags by given ones

Tag validation: only allow 0..9a..zA..Z- (specifically do not allow blank or comma)

[ThomasWaldmann]

borg info --json <archive> can be used to write tests for this.

Later, to give the tags better visibility, borg repo-list's default format should grow a {tags:10} placeholder and the ArchiveFormatter class needs to support getting the tags from the Archive and formatting them as TAG1,TAG2,... (sorted).

[ThomasWaldmann]

Selection based on tags for miscellaneous borg operations: see #8423.

[ThomasWaldmann]

I decided to do the low-level groundwork for this in #8426, so anyone taking this issue only needs to deal with the Archive.tags data structure (a set).

[ThomasWaldmann]

Fixed by #8449.