Add BLU Gateway BTHome to MQTT script

[luismiguelbp]

Summary

• Adds a Shelly BLU Gateway script that decodes BTHome advertisements and publishes Shelly Gen2-style MQTT RPC notifications.
• Supports MQTT publishing without requiring native RPC notifications over MQTT.
• Includes Espruino-compatible helper logic without RegExp usage.

Tested

• Tested on Shelly Bluetooth Gateway SNGW-BT01 with MQTT enabled.
• Confirmed decoded BLU Door/Window events publish to <topic_prefix>/events/rpc.
• Confirmed payloads include decoded fields such as battery, illuminance, window, rotation, button events, RSSI, and inferred device type.