Allow mixins of ModuleOptionsMetadata

A lot of modules have similar options. Moreover, job modules often have options that belong to at least two domains (eg jdbc + hdfs).

I think that by using FlattenedCompositeModuleOptionsMetadata, we could come up with a way to combine several options POJOs into one. Something like:

public class JdbcHdfsOptionsMetadata {

@OptionsMixin
private JdbcOptionsMetadata jdbc;

@OptionsMixin
private HdfsOptionsMetadata hdfs;
}

this would expose eg "driverClass" as well as "rolloverSize" as top level options. Values could be actually injected into the fields, so that eg custom validation could occur (default validation for the mixin class would occur by default)