Date time overhaul (#1139)

Working on overhauling pgrx's `Date`, `Time`, `TimeWithTimeZone`, `Timestamp`, `TimestampWithTimeZone`, and `Interval` types.

We now delegate most everything back into Postgres so we're no longer responsible for re-implementing date/time math which is impossible to get correct.

The various types can now actually be constructed, and the ones `with time zone` support construction at a specific timezone and also being shifted to a specific timezone.

I've also ripped out support for the `time`  crate -- the code around that is too much highly-specific knowledge between both Postgres and `time` in order to confidently maintain going forward.  Users that use that will need to make their own wrappers to and implement `time` support themselves.

Some key changes and features are:

- [x] Better error handling -- need to invent a custom error type instead of the generic `PgSqlErrorCodes`
- [x] More `impl From` conversions between the types
- [x] `impl Add/Sub`
- ~~`impl Index` if possible so that `let hour = timestamp[DateTimeParts::Hour];` ~~ not possible
- [x] Drastically simplify the `Serialize` implementations and also properly implement the corresponding `Deserialize` trait
- [x] doc comments
- [x] additional unit tests
- [x] top-level constructor functions like `now()`, `current_timestamp()`, etc. 
- [x] misc. functions from https://www.postgresql.org/docs/15/functions-datetime.html like `age` and `date_trunc`
- [x] an example that shows off some of the simple usage

.github/workflows/tests.yml

@@ -191,6 +191,9 @@ jobs:
     - name: Run custom_sql example tests
       run: cargo test --package custom_sql --features "pg$PG_VER" --no-default-features
 
+    - name: Run datetime example tests
+      run: cargo test --package datetime --features "pg$PG_VER" --no-default-features
+
     - name: Run errors example tests
       run: cargo test --package errors --features "pg$PG_VER" --no-default-features