Allow PDK users to configure a allowlist of ENV vars to be passed along to subprocesses

[scotje]

Currently, PDK carefully manages the ENV vars when it invokes a subprocess to ensure the subprocess is executing in the correct Ruby/Bundler context, etc.

We should consider allowing users to configure a allowlist of additional ENV vars that they would like PDK to pass through to subprocesses to support things like the LD_PRELOAD proxy use case or as a mechanism for passing along authentication credentials.

If the user includes ENV vars in this whitelist that are known to be problematic (such as Ruby load path) we should consider issuing a warning.