Added list of env variables for fn-plugins

KRM-functions should be able get env variables.
Reflecting in `build` cmd args added in [1]
for `fn run` cmd.

[1]
https://github.com/kubernetes-sigs/kustomize/pull/2988/

api/types/pluginrestrictions.go

@@ -53,4 +53,6 @@ type FnPluginLoadingOptions struct {
 	NetworkName string
 	// list of mounts
 	Mounts []string
+	// list of env variables to pass to fn
+	Env []string
 }