feat: switch to UTC times for file metadata in Autonomi public and private archives created by dweb

[happybeing]

Autonomi Archives currently hold file metadata using local system time, but I plan to ignore that and switch to using UTC so that anyone anywhere can know the true times for file modification and creation.

I've suggested this also be adopted for Autonomi official docs and implementations (e.g. in ant-cli) in issue

If someone wants to take this on and submit a PR for dweb (and ideally ant-cli) please go ahead.