ngclient: Be better with concurrent instances

Users may run multiple updaters at the same time -- it's not useful but it can happen with longer dependency chains like https://github.com/sigstore/sigstore-python/issues/1403: model-signing uses sigstore-python which uses python-tuf...

Currently we don't handle the potential conflicts WRT reading and writing metadata and target files to local cache that result from concurrent updaters. This is especially an issue with windows where concurrent file access leads to issues more often but it can be problematic in linux too -- this is why currently we just document Updater as "should be a singleton".

Possible improvement: lockfile

I think it makes sense to not try to solve this for just a single process but multiple real processes running updaters (something I can easily imagine happening in real life). In that case I think a lock file per repository would be best we can do:

• check for lock file before reading/writing
• create lock file, read/write, remove lock file

The reason this hasn't been done is that lockfiles are a pain to do especially cross platform.

Some open questions:

• what to do when a lock file exists -- log a warning and wait (for how long?)? I don't usually like when libraries wait for things but maybe this works? What if the lock file is stale (let's say hours old)?
• manually handling lockfiles in a way that works on windows is painful... but we've also done a lot of work to avoid unnecessary dependencies in this project. Using a dependency for this is likely the smaller evil
• when exactly do we lock?
  • locking for lifetime of Updater probably has annoying side effects and is not expected by users
  • locking for duration of refresh() and get_targetinfo() seems logical at least at first glance. Locking for duration of download_target() is probably less important but could be done

Possible improvement: global state

If we are not interested in multiple processes but just multiple threads with updaters, we could always add some global state tracking... It sounds quite unappealing but possible. I feel like I'd rather advice python-tuf users to use ngclient Updater as a "per-repository-singleton"?

CC @spencerschrock, let me know if you have thoughts

Start by reading the ngclient Updater flow, especially refresh(), get_targetinfo(), and download_target(), and trace where metadata and target files are read or written in the local cache. Define and document a cross-process concurrency policy for lock creation, waiting, stale locks, and lock scope; the issue is done when those open questions have an agreed implementation path.