Add support for List in boilerplate code

[pierre]

See

killbill-plugin-framework-java/src/main/java/org/killbill/billing/catalog/api/boilerplate/UsageImp.java

Line 310 in c0f16f8

public T withTiers(final Tier[] tiers) {

Consider both public T withTiers(final Tier[] tiers) and public T withTiers(final List<Tier> tiers).