Paginated API call support

[aik099]

The JIRA REST API supports pagination ( see https://developer.atlassian.com/cloud/jira/platform/rest/v2/intro/#pagination ).

We've implemented it only for issues (the Api::search method has $start_at and $max_results parameters and Walker class makes use of them).

There are many more API methods in JIRA (see https://developer.atlassian.com/cloud/jira/platform/rest/v2/ ) , that also use pagination, and if not handled properly, then the developer needs to write lots of boilerplate code to handle it manually.

Needs to create a universal walker (probably callback-based) that can iterate any API response, that is paginated.