[Feature #20707]

Converting Time into RFC3339 / ISO8601 representation is an significant
hotspot for applications that serialize data in JSON, XML or other formats.

By moving it into core we can optimize it much further than what `strftime` will
allow.

```
compare-ruby: ruby 3.4.0dev (2024-08-29T13:11:40Z master 6b08a50a62) +YJIT [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-08-30T13:17:32Z native-xmlschema 34041ff71f) +YJIT [arm64-darwin23]
warming up......

|                        |compare-ruby|built-ruby|
|:-----------------------|-----------:|---------:|
|time.xmlschema          |      1.087M|    5.190M|
|                        |           -|     4.78x|
|utc_time.xmlschema      |      1.464M|    6.848M|
|                        |           -|     4.68x|
|time.xmlschema(6)       |    859.960k|    4.646M|
|                        |           -|     5.40x|
|utc_time.xmlschema(6)   |      1.080M|    5.917M|
|                        |           -|     5.48x|
|time.xmlschema(9)       |    893.909k|    4.668M|
|                        |           -|     5.22x|
|utc_time.xmlschema(9)   |      1.056M|    5.707M|
|                        |           -|     5.40x|
```