[Python] Proposed support for additional client 'HTTPX' via Postman Code Generators

[M-NK-Y]

To whom it may concern,

Having used postman for almost two years now the utility of the code generator has always proved itself useful and still does to this day, 'Requests' and 'http.client' as modules are without question as important to the ecosystem as they come but as time drags on their age begins to show.

With such a wealth of talent being invested by teams behind projects such as 'HTTPX' with flashy new gadgets like H2 support, among other timeless classics such as support for asynchronous usage it feels long overdue to consider the world beyond that of 'Requests' and the dated 'http.client', and that's exactly what this PR seeks to do.

Fundamentally the support for 'HTTPX' within Postman as an option for code generation is the primary motivation for this PR but secondary to this; you'll find also some added flair to existing logic that seeks to address and modernize features like HTTP query string parsing and JSON-as-an-argument handling in addition to a prototype for a structural rework in how given snippets are actually generated.

Please go easy on me, this is my first ever public PR and whilst I believe I've satisfied all the Newman tests it's highly possible that I've missed something so don't hesitate to mention if something's gone wrong and needs to be revised.

Thanks for reading, and I hope once you have a chance to check it out you'll feel the same way:)