Add ability to provide env vars to plugins

[calvn]

Fixes #5177

We could add a field of allowed pass-through environment variables to the plugin catalog. Then you can specify which ones should be passed through to the child process. Or, we could allow directly specifying key-value pairs. Or both (e.g. we could have passthrough_env and additional_env or so).

Only the former of the two options is implemented. The reasoning is the same behind separating the command and args params, which started off as being one and the same before getting split (and tagging the 2-in-1 option as deprecated). If users end up asking for the ability to pass in key=value pairs as part of the command then we can add it later, but this is more in line with how exec.Command works.

[briankassouf]

👍 looks great! Can you also add the docs changes to this PR?

[calvn]

Sure thing! I also plan on refactoring/combining TestAddTestPlugin and TestAddTestPluginTempDir before merging this PR.

[jefferai]

Looks great!