Backport ANCM environment variable additions

[BrennanConroy]

Description

In order to run apps locally in VS with a nice. clean experience, there are times when VS will modify a users web.config, which is not a nice experience. In order to avoid modifying user code we have added some features to ANCM to allow VS to use environment variables to override behavior instead of modifying users' files.

#19451
#19450

Customer Impact

This issue has been reported by customers, it impacts them because it modifies their files which is very undesirable behavior.
See this issue https://github.com/aspnet/websdk/issues/564 with over 20 upvotes.

Regression?

No.

Risk

Low, we look for the new environment variables but fallback to old behavior if they aren't present.

[ghost]

Hello human! Please make sure you've included the Shiproom Template in a comment or (preferably) the PR description. Also, make sure this PR is not marked as a draft and is ready-to-merge.

[analogrelay]

I won’t be able to be at Tactics to rep this (childcare strikes again!), I can send an email with more details and justification if necessary. This is a request from VS Web Tools. Currently VS modifies the web.config to inject their launch configuration data but this breaks some customer scenarios (and in general, modifying a file that customers maintain causes weird conflicts).

In 5.0 we’re introducing new environment variables that VS can set when launching the app to override the web.config values it normally modifies. Due to high customer feedback (both in our repo and VS feedback) we want to back port this to 3.1. 2.1 is not affected since this only applies to IIS in-proc hosting (added in 2.2).

[analogrelay]

Marking approved as per email discussion.

[BrennanConroy]

Want to click the merge button too :D

[analogrelay]

Yep, just wanted to check we were open for merging. And we are! Yay!