Setting variables and running commands

[Aram Peres]

Add support for setting variables and running commands on UPS devices.

For security reasons this should be gated behind a new crate feature.

Docs:

• SET: https://networkupstools.org/docs/developer-guide.chunked/ar01s09.html#_set_3
• INSTCMD: https://networkupstools.org/docs/developer-guide.chunked/ar01s09.html#_instcmd_3