Testing: Add type annotations to transfertool, mock and fts3

[rdimaio]

Part of #6588.

This PR is a bit larger, the main bulk is fts3.py. The abstract methods defined in transfertool.py largely have the same type hints in their implementations in mock and fts3, with a few exceptions in cases where the sub-classes modified the types expected in the methods.

On the pyright failure:

 Found 1 new problems in /home/runner/work/rucio/rucio/lib/rucio/daemons/conveyor/receiver.py
  - 1 errors with message """Argument of type "Unknown | None" cannot be assigned to parameter "session" of type "Session" in function "get_db_fields_to_update"
                               Type "Unknown | None" is incompatible with type "Session"
                                 "None" is incompatible with "Session"""".
    Candidates: line 86

This is an issue within the Conveyor Receiver where we're passing a session that might be None to get_db_fields_to_update. I didn't want to potentially impact the functionality of this daemon, so I didn't add a check to ensure that session exists before passing it, but I think this check should probably exist in the daemon. I think it's best to keep this out of here to limit the scope of the PR to transfertool, but we might want to add it in the future.