Add --start time support to sync

[scelfo]

I would like to add support for passing a --start time to the sync operation.

This would make it possible to implement simple offsite storage by syncing only the last day or even the last 10 minutes.

My video recorder has much more storage capacity then what I'd like to sync offsite so I don't want to have sync start at the oldest content on unifi protect.

I'll be happy to send a pull request to implement this but I wanted to file this to track the work.

[danielfernau]

Sounds good, feel free to open a pull request once you're ready and we can have a look at it. Having this as an alternative to just using the timestamp of the earliest recording available on the Protect console would for sure be helpful in some scenarios.