Integrate api pojos

[sbrossie]

This task consists in leveraging the api-pojo code generation tool to generate all the POJOs associated with our killbill-api interfaces, and check them in as part of this repo.

Then, go through this code and check what existing POJOs we had created by hand and either replace or extend (if there is some business logic) those. Doing a quick inspection, I see the following candidates - to be confirmed:

PluginDryRunArguments
PluginTag
PluginEntitlementSpecifier
PluginPaymentOptions
PluginPlanPhasePriceOverride
PluginCustomField
PluginAccountData
PluginPaymentTransactionInfoPlugin
PluginHostedPaymentPageFormDescriptor
PluginPaymentMethodPlugin
PluginGatewayNotification
PluginPaymentMethodInfoPlugin
PluginCallContext
PluginTenantContext
PluginInvoiceItem

[sbrossie]

This work was done as part of #59 and #61 and is currently in work-for-55 branch until we decide if this goes in master (0.22.x) release or active work-for-release-0.23.x branch.