Allow Platform-specific Custom Power Operations

[sisungo]

Introduction

Some platforms support power operations that aren't supported by other platforms and can be integrated into Airup's architecture for system stability, like Linux's kexec.

Unresolved Questions

• How should the API be designed?
• The feature introduces a new error kind which is caused by parse failure of the command string. How to design the API to avoid terminating airupd process from failure? Should we use std::io::Error or our custom error type?
• How should the public RPC API be designed?