feat: storing postman collections as directories + newman enhancements

[knutties]

Single file postman collections are hard to manage in repositories. We propose a format change where Postman collections are split into smaller files and managed within their own directories. This PR includes changes in newman command line tool to manage collections that are stored as directories. We believe this approach helps in the following:

1. A cleaner and easier way to review pull request diffs when collection files are stored in source control systems.
   • example of a hard to review change to a collection file: https://github.com/juspay/hyperswitch/pull/1948/files
2. Makes newman a comprehensive CLI companion to Postman - going beyond just running tests to creating and managing collections.
3. Enable developers to use their favourite code editors to add/remove new tests.

A more detailed note on this implementation is contained in this document that is part of the PR.

We have started leveraging this fork in our Open Source Payment Orchestrator Platform (Hyperswitch) at Juspay and Postman based test maintenance has become a lot simpler.

I am eager to get feedback from the owners to see if this will be a welcome enhancement to the newman toolkit.

Thanks !

PS: please do not mind the un-squashed commits. Have left it around for easier reviewing.