Allow setting the base parameter of indexColumn

Hibernate allows a [`base` attribute](https://docs.jboss.org/hibernate/orm/4.3/manual/en-US/html/ch07.html#collections-indexed) (or `@ListIndexBase` annotation) for List collections, to map legacy databases having the list index starting from 1.

I couldn't find a way to do the same in Grails. I believe it should be added to the ColumnConfig class and then to the code producing the Hibernate mapping.

Start by locating ColumnConfig and the code producing the Hibernate mapping for indexed List collections. Read how indexColumn is currently configured, then trace the generated mapping and compare it with Hibernate's base attribute or @ListIndexBase support. Done means Grails can configure a nonzero list-index base for legacy databases and the generated mapping reflects that setting.

Written by the indexing model from the issue text.