Compatibility with chrono via feature flag

[t3hmrman]

Thanks for this awesome project, huge fan and user of pgrx!

Often when working with date-times and other temporal Postgres types, I often end up converting to chrono native types -- it would be nice if pgrx had a chrono feature flag that exposed some simple From/TryFrom/etc implementations.

I'd be willing to work on this if it was something ya'll would be willing to consider

[workingjubilee]

That seems reasonable, the only caveat I will note is that our current implementations for these datetime types don't adequately bound themselves in some cases, but I intend to fix that Somewhat Soon.

• Proptest datetime types like Date #1525

Postgres offers pretty generous limits but they do exist, and offers different precisions. This makes converting to/from Chrono types somewhat error prone. So I "most want" these in the case where there is

• no precision loss when converted into the Postgres type
• no out-of-bounds values when converted into Postgres type

[t3hmrman]

Hey @workingjubilee I've got a draft PR of one conversion (pgrx::datum::Date -> chrono::NaiveDate) and would appreciate some early feedback if you can find some spare time!

#1603

These tests are certainly a good candidate for proptests, the test that's in there right now is just a simple static test -- I can certain make sure that these are proptested while I'm here.