Add reflect-config.json for GraalVM native-image

[alexarchambault]

This file makes it easier for directories-jvm users to generate GraalVM images. It marks java.util.Base64 and java.util.Base64$Encoder as possibly obtained / inspected by reflection, as directories-jvm does now.

If it's not done by directories-jvm, users have to

• first discover, after a possibly lengthy image generation, that these classes are reflected upon, and
• add these to their GraalVM config files or command-line options.

Whereas if directories-jvm ships that file, they don't have to think about that.

I didn't add tests right now, but if I take the time to look into #32, maybe a native binary could be generated for that CLI, which would ensure there's no regression in reflect-config.json.

The additional settings could be possibly narrowed... I didn't take the time to do it.

[soc]

Thanks!