Support multiple index statements in Field-definition

[thomasht86]

Currently, the schema.txt template is hardcoded to:

index: {{ field.index }}

which means we are unable to define fields like eg. this:

field predicate_field type predicate {
            indexing: attribute
            index {
                arity: 2  # mandatory
                lower-bound: 3
                upper-bound: 200
                dense-posting-list-threshold: 0.25
            }
        }