invoke: Enable plugin file names with extensions

invoke: Enable plugin file names with extensions

A CNI network configuration file contains the plugin's executable file name.
Some platforms like Windows require a file name extension for executables.
This causes unnecessary burden on admins as they now have to maintain two
versions of each type of netconfig file, which differ only by the ".exe"
extension. A much simpler design is for invoke package to also look for
well-known extensions on platforms that require it. Existing tests are
improved and new tests are added to cover the new behavior.

Fixes #360