Add from_timestamp_millis to DateTime<Utc>

[xmakro]

As discussed in #264 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5536687) 91.68% compared to head (d840420) 91.69%.

Additional details and impacted files

@@            Coverage Diff             @@
##            0.4.x    #1374      +/-   ##
==========================================
+ Coverage   91.68%   91.69%   +0.01%     
==========================================
  Files          38       38              
  Lines       17586    17608      +22     
==========================================
+ Hits        16123    16145      +22     
  Misses       1463     1463              

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[djc]

Do you have an actual use case for the micros variant?

[xmakro]

I don't need it. I added it consistency but happy to remove it. In my case I serialize DateTime<Utc> with timestamp_millis and then restore it with timestamp_from_millis. If I needed more precision, I might have chosen timestamp_micros.

[djc]

Okay, I would prefer to defer the addition of micros in that case until someone comes up with a use case for it. IME millis timestamps are much more common than micros timestamps since they're used a bunch in the JS ecosystem.

[xmakro]

Sounds good. I changed the PR to only include timestamp_from_millis.

[djc]

Thanks!