[Release/3.1] Port profiler APIs to set and retrieve environment variables to 3.1

[davmason]

This ports #27157 to 3.1.

Issue
On linux/unix/mac platforms the PAL creates a cache of the system environment variables at startup since accessing environment variables is not thread safe. This means that any call the putenv or getenv by native code after process startup will be ignored in managed code or any child processes spawned by managed code.

Profilers often use environment variables to communicate with child processes or managed code, historically on Windows they could just call SetEnvironmentVariable and it would be picked up, but now on cross plat scenarios the only workaround is to use IL rewriting to inject a call to Environment.SetEnvironmentVariable, which is a substantial amount of work if the profiler is not already doing IL rewriting.

Customer impact
Without this fix profilers won't be able to coordinate with child processes while debugging. See this comment by the profiler author requesting this be included in 3.1:

@davmason Without this, we can lost all of our features related to child processes: 1) ability to profile child processes (for details see https://github.com/dotnet/coreclr/issues/23383#issuecomment-514587669), 2) ability to take snapshots automatically depending on child processes hierarchy, 3) unit-testing profiler support. Most likely, end user experience will be significantly worsened if our "tricks" stop working.

Originally posted by @k15tfu in #27157 (comment)

Fix description
Expose the PAL's implementation of SetEnvironmentVariable and GetEnvironmentVariable on the profiler interfaces.

Risk
There is very low risk (essentially no risk) to existing codepaths since the existing codepaths are unchanged. This fix only exposes new functionality and does not modify existing behavior.

[MeiChin-Tsai]

approved for 3.1. For customer unblocking.

[k15tfu]

@davmason Do you need anything from me?

[davmason]

@k15tfu nothing is needed, it was approved and I'm waiting merge until the branch is open for checkins. The release branch is currently locked down waiting for the preview release to build

[k15tfu]

@davmason BTW, how do you generate corprof.h without all of these extra spaces I have? I use midl src\inc\corprof.idl from VS2017 developer console.

[davmason]

@davmason BTW, how do you generate corprof.h without all of these extra spaces I have? I use midl src\inc\corprof.idl from VS2017 developer console.

If you build on windows midl will run automatically and you can grab corprof.h out of the obj folder, but there's no way around the whitespace issues. I don't know why midl constantly changes whitespace but it does.

[k15tfu]

@davmason Is there any ETA for 3.1-preview3?