Support iterating over Tabs

[nextrevision]

The current com.docusign.esign.model.Tabs model contains getters and setters for all the tab types. There doesn't appear to be a good way to determine which tabs are defined without a verbose if/else chain or using some other sort of hacky solution like reflection. This is generally fine for creating tabs, but inspecting tabs becomes a chore.

Is there a good way to support iterating over Tabs to determine which ones are set? Is there merit to having a base tab type that implements some subset of fields which the other types extend?

[mmallis87]

@nextrevision apologies that no one responded till now. Unfortunately the way our SDKs are created (generated from a Swagger v2 spec file) don't allow us to tackle this in the near term. That's said I created ticket so we can prioritize it.