Add new functions for manipulating the environment

[mjcarroll]

This adds functions for using collections with the environment. It allows for conveniently retreiving and setting the environment variables via a map, rather than individually.

This also adds a clearenv and printenv for completeness.

[codecov]

Codecov Report

Merging #114 (b9fc005) into gz-utils2 (c918355) will decrease coverage by 12.31%.
Report is 13 commits behind head on gz-utils2.
The diff coverage is 72.24%.

@@              Coverage Diff               @@
##           gz-utils2     #114       +/-   ##
==============================================
- Coverage      91.97%   79.67%   -12.31%     
==============================================
  Files              6        8        +2     
  Lines            137      364      +227     
==============================================
+ Hits             126      290      +164     
- Misses            11       74       +63     

Files	Coverage Δ
cli/include/gz/utils/cli/GzFormatter.hpp	88.13% <ø> (ø)
include/gz/utils/Subprocess.hh	96.61% <96.61%> (ø)
src/Environment.cc	87.03% <81.81%> (-8.21%)	⬇️
include/gz/utils/detail/subprocess.h	59.25% <59.25%> (ø)

[iche033]

Is the idea to eventually deprecate the env functions in gz-common and use these instead?

[mjcarroll]

Is the idea to eventually deprecate the env functions in gz-common and use these instead?

The gz-common ones are now just forwarding to the other env functions here. I added them because we have packages that we want env functionality in that don't depend on gz-common (eg sdformat).

[azeey]

A couple of tests are failing on windows.

[mjcarroll]

A couple of tests are failing on windows.

Neat. So what this was is that cmd.exe injects an extra environment variable =C: which powershell does not. This interfered with my splitting logic.

[azeey]

LGTM!