Make confined_target_dirs optional field in the MIRROR_SCHEMA

Description of issue or feature request: I was thinking about why confined_target_dirs is not an optional field in the MIRROR_SCHEMA. https://github.com/theupdateframework/tuf/blob/fdccb8dc0bf34e2b016ce20f1570e582dcd459e7/tuf/formats.py#L408

Given that [''] matches everything and [] matches nothing this API can be confusing and lead to a mistake. For me, it sounds more logical to not provide confined_target_dirs list at all if I want to accept queries for all targets for that mirror and provide a list of confined_target_dirs if it’s otherwise.

Current behavior: confined_target_dirs is a mandatory field in the MIRROR_SCHEMA.

Expected behavior:

confined_target_dirs to be an optional field in the MIRROR_SCHEMA.

[mnm678]

We should just remove it. Nobody uses it AFAIK.

For backwards compatibility, we can make it optional for now, then remove it in a future major version release.

[MVrachev]

Do you @mnm678 and @trishankatdatadog think we can change the API for confined_target_dirs key in the MIRROR_SCHEMA? Instead of using [''] to allow all targets that we can change the documentation and check that if confined_target_dirs does exist for MIRROR_SCHEMA and if it doesn’t it means that all targets are allowed.

Of course, it’s good if we don’t remove the checks if confined_target_dirs is [] because of possible clients using it the older way.