Treat CACHEDIR.TAG as a default value for exclude-files-attributes

[vlsi]

• I have searched open and closed issues for duplicates.
• I have searched the forum for related topics.

---

Description

I suggest Duplicati to respect CACHEDIR.TAG by default, so the users don't have to configure it every time.

See

• Support marker file for excluding directories #1482 (comment)
• https://forum.duplicati.com/t/can-i-make-a-backup-respect-cachedir-tag/924

Currently, there's --ignore-filenames option, however, users need to specify it explicitly.

It would be nice if CACHEDIR.TAG was added to --ignore-filenames by default

[kenkendk]

I have made a PR that adds this. It does not fully comply with the specifications, as we would need to read the file and check the header signature.