[BFW-5760] Prusalink: Add support for setting ready state via api

[bkerler]

This solves issue #3640 and implements setting ready state via rest api.

Examples :

# Get ready status
GET /api/v1/ready
# Response: {"ready": true}

# Set printer as ready
PUT /api/v1/ready
# Response: 204 No Content (or 409 Conflict if not possible)

# Unset printer ready
DELETE /api/v1/ready
# Response: 204 No Content

# Check status includes ready state
GET /api/v1/status
# Response includes: "printer": {"state": "READY", ...}

Implementations in Prusa-Link-Web are prepared, but optional : prusa3d/Prusa-Link-Web#526