ngclient: persist metadata in a safer way

tuf/ngclient/updater.py: Updater._persist_metadata():

    with open(os.path.join(self._dir, f"{rolename}.json"), "wb") as f:
        f.write(data)

this is not as safe as it should be (as file could be corrupted midway through). This should work:

• write content to tempfile
• use sslib_util.persist_temp_file() to move tempfile to correct filepath

[suvadityamuk]

Understood, I’ll make the changes accordingly. Could you possibly assign this task to me?

[suvadityamuk]

Hey. I have prepared a fix to this. Should I put up a pull request on this? It’s my first open-source contribution, so any inputs will be much appreciated.