ENH: initial support for non-nano Timestamp
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> So just to confirm, this PR just allows preserving the resolution from np.datetime64 objects?

Correct.  This way I can incrementally implement tests for tznaive cases, then expand to tzaware cases, etc.

> Do you have plans to support passing in resolution from the Timestamp constructor too?

Yes, though the exact API is still up in the air (xref #46587). On last week's dev call we were leaning towards a new keyword, though im still reticent, so punting on this for the time being. @jreback gentle ping; nothing time-sensitive, but many more steps to go (ditto #46847, #46828, #46811, #46803)