Add a fs watcher based reloader for PathOrContent

[douglascamata]

Signed-off-by: Douglas Camata [email protected]

Extracting the filesystem based reloader for PathOrContent from https://github.com/thanos-io/thanos/pull/5673 into the extkingpin package here so that it can be more easily reused by other projects and even in Thanos itself (where it was originally created).

[douglascamata]

@saswatamcode suuuure, working on it. 💪

[douglascamata]

@saswatamcode when do you think we can merge this? 😬

[douglascamata]

@bwplotka this PR is now severely outdated. I made the reloader independent of fs watchers due to problems with k8s secrets and configmaps (see https://github.com/thanos-io/thanos/pull/6503). Will update the PR here when I get some time.