Fix execution of plugins on Windows

[welteki]

Description

• syscall.Exec is not supported on Windows systems. Use the os/exec package instead.
• On windows the plugin executables should have the .exe filename extension in order to execute them.

Motivation and Context

• I have raised an issue to propose this change (required)

Executing plugins on Windows failed with the error Error from plugin: not supported by windows.

How Has This Been Tested?

• Verified faas-cli plugin get pro works on windows and stores the executable in the correct path $HOME/,openfaas/plugins/pro.exe

• Verified faas-cli plugin get pro still works Linux systems.

• Verified running plugins works on Windows systems.

• Verified running plugins still works on Linux systems.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.