feat(UnionConverter): Delegate to STJ 11 intrinsics

https://github.com/dotnet/runtime/pull/125610 produces very similar output

Key interop issues are:
- builtin tag is `$type`; default for `UnionConverter` is `case`. Ideally there would be a way to have both be honored on read
- ability to supply a case name that'll be used as catch-all (with optional `JsonElement` body)

The default OOTB behavior is very similar to the two autoUnion opt ins being applied (minus the discriminator tag name)

It may be worth considering having a way to upconvert reading from `case` while writing as `$type`
Also need to visit #116, #129, #117 which are likely at least partially covered by this too