Improve DateTime{Offset} "r" and "o" formatting performance (#17092)

Improve DateTime{Offset} "r" and "o" formatting performance (#17092)

Two main changes:
1. Rewrote the formatting to use span, only to then discover that we already had almost exactly the same implementation in Utf8Formatter.  As that one had some extra optimizations around JIT behaviors, I ported that over instead.
2. Avoided [ThreadStatic] lookups unless necessary.

ToString/TryFormat for "o"/"O" improve by ~2.5x.

ToString/TryFormat for "r"/"R" improve by ~3x.