File system watcher deadlock

[fgiorgetti]

Describe the bug
A deadlock has been identified during the review of #2548 .
If the FileWatcher.Add method gets called repeatedly, it falls into a dealock.

How To Reproduce

Run the new unit test (on the upcoming PR):

go test -count=1 -v -run TestFileWatcher_Add ./internal/filesystem/

Expected behavior
Test passes (no deadlock).

Environment details
Impacts skupper 2.2.1.