Add timestamp_micros to Utc

[emikitas]

I wanted to know if there is a reason for having the functions timestamp_millis and timestamp_nanos in the trait TimeZone but not timestamp_micros.
If there is no reason not to have it, will it be possible to implement it?

[djc]

What's your use case for it?

[emikitas]

I wanted to add support for processing microseconds in VRL, specifically in the following function:
https://github.com/vectordotdev/vrl/blob/000109385569314c6715300bfd84cba0971646c3/src/stdlib/from_unix_timestamp.rs#L5
I figured I might not be the only person interested in transforming microsecond timestamps into DateTime, so I figured I could add it to chrono.
I'd like to implement it myself, but I first wanted to check if there is a reason not to.

[pitdicker]

timestamp_millis was added in #268.
Personally I am not a fan of the large number of timestamp_* methods we are getting. But one more timestamp_micros (that returns a LocalResult like timestamp_millis_opt) would make the set complete 🤷.

Maybe, if you add it: can you also change TimeZone::timestamp_millis_opt to use NaiveDateTime::from_timestamp_millis under the hood? So that we don't have the same functionality with two different implementations.

[djc]

Okay, please submit a PR.