Add a python/exec payload to execute OS commands

[zeroSteiner]

This adds a single ARCH_PYTHON payload that is capable of executing arbitrary OS commands. It uses subprocess.Popen to execute commands through a shell environment as defined in the docs. Tested with Python 2.7, 3.4 and 3.12.

Verification

• [ ] Start msfconsole
• [ ] use payload/python/exec
• [ ] set CMD somecommand (try and trick it with quotes and what not, make sure it works as intended)