Allow setting parent environment explicitly

[davidlatwe]

Feature Propose

Allow to provide unified application launching environment from allzparkconfig.py.

Motivation

Allzpark can be launched in various way, e.g. from a Python virtual environment, or a Rez resolved context. And Allzpark can be backed with bleeding-rez or nerdvegas/rez, which has a bit different base environment control scheme between two approaches.

To overcome those differences and other surprises, allowing subprocess (application) parent environment to be set in allzparkconfig.py would be the most straight forward and effective solution.

Additional Features

• Add "Parent" tab in "Environment" dock page to show current parent environment.
• Add "Diagnose" tab in "Environment" dock page to show actual subprocess environment.
• Add "Code" tab in "Context" dock page to show script that is used to spawn shell from resolved context.

[mottosso]

Wow, this is great. :D Get Shell Code is brilliant, it's one of those obscure things about Rez you don't really think about but should really know, and want to know once you start running into odd behaviors with the result. Great job, I have no notes.

[davidlatwe]

Thanks @mottosso :D
Merging this today 🚀