Migrate datetime types into pgrx::datetime (pgcentralfoundation#2199)

Everything remains accessible from the previous paths, so this is purely
organizational for now with no end-user impact except new path options.

Author: workingjubilee

pgrx/src/datetime.rs

@@ -0,0 +1,17 @@
+pub mod support;
+pub(crate) use support as datetime_support;
+pub use support::*;
+
+mod date;
+mod interval;
+mod time;
+mod time_stamp;
+mod time_stamp_with_timezone;
+mod time_with_timezone;
+
+pub use date::*;
+pub use interval::*;
+pub use time::*;
+pub use time_stamp::*;
+pub use time_stamp_with_timezone::*;
+pub use time_with_timezone::*;