Support plugin `.env` file

[nonsocode]

.env is a well-known format of describing environment variables. Support for this would greatly reduce the scripting overhead needed to run multiple plugins that share the same (or different) sets of environment variables. An option like --plugin-env-file could be used (similar to --plugin-env)

At runtime, sonobuoy could create a config map that will be added to the plugin pod's spec's env field.
Alternatively, sonobuoy could parse the content of the file and create environment entries on the Pod's spec like it currently does with --plugin-env