Add Required attribute to ManPageGenertor

[epugh]

I am working on migrating to picocli from commons-cli for Apache Solr. One of the exciting enhancements is generating the documentation for commands.

We ran into a problem that in the options documentation we don't distingush required versus optional:

You can see the [ ] work for the synopsis, but not in the individual option docs.

picocli's `ManPageGenerator.writeOption()` ignores `option.required()` entirely. The only signal is the synopsis bracketing — and the synopsis is easy to miss/ignore once you scroll past it.

It would be good to add the option.required() attribute!

[epugh]

I did some Gradle coding to work around this: apache/solr#4433