Only enabling draft202012 features misses some features implemented only for draft201909

[jrudolph]

In my case, it was the uuid format validator that is only available when draft201909 is enabled.

One possibility could be to include the full draft201909 feature in draft202012 in cargo.toml.

Alternatively, if draft202012 is not a strict superset of draft201909, the conditional compilation should be changed in a way that all json schema features that are both in draft202012 and draft201909 are also enabled if only the draft202012 is selected.

I can open a PR if we can agree on which way to go.

Thanks for this nice library!