Increased flexibility generating MQTT topics from Xiaomi Mijia BLE Sensors

**Describe the problem you have/What new integration you would like**
I cannot get my required topic structure for MQTT from the Xiaomi Mijia BLE Sensors functionality
Topics are generated thus -:[topic prefix]\[device name] where the device name is a single string for a device's measurement entry in the config..
So in my example
Device strings are "Xiaomi27 Temperature", "Xiaomi27 Humidity", "Xiaomi3D Temperature", "Xiaomi3D Humidity"
I get topics "[topic prefix]\Xiaomi27_Temperature", "[topic prefix]\ Xiaomi3D_Temperature" etc

The ability to specify topics by sensor measurement within device (At the moment it is sensor measurement AND device
To get topics like this "[topic prefix]\Xiaomi27\Temperature", "[topic prefix]\Xiaomi27\Humidity", "[topic prefix]\Xiaomi3D\Temperature" etc

**Please describe your use case for this integration and alternatives you've tried:**

An MQTT home automation platform. Would allow me to parameterise and manage devices and measurements
I tried putting "\" in the name, but these were just ignored -:)

**Additional context**

None

Start by tracing the Xiaomi Mijia BLE Sensors functionality that generates MQTT topics from device measurement names. Define the configuration and topic-generation behavior needed to produce separate device and measurement segments, then verify the resulting topics match the examples for Xiaomi27 and Xiaomi3D.

Written by the indexing model from the issue text.