Add additional level in mqtt hierarchy for esp32_ble_tracker (might be useful for other applications as well)

**Describe the problem you have/What new integration you would like**

I am using the esp32_ble_tracker to collect data from multiple ble sensors. The sensors have multiple values. I would like to improve the layout of the mqtt messages by adding an extra layer in the mqtt label hierarchy using the sensor name.

Currently it look like this:
/node-name/sensor/atc_temperature_bathroom
/node-name/sensor/atc_humidity_bathroom
/node-name/sensor/atc_battry-voltage_bathroom
/node-name/sensor/atc_battry-level_bathroom
/node-name/sensor/atc_temperature_sleepingroom
/node-name/sensor/atc_humidity_sleepingroom
/node-name/sensor/atc_battry-voltage_sleepingroom
/node-name/sensor/atc_battry-level_sleepingroom

The desired structure is:
/node-name/sensor/bathroom/atc_temperature
/node-name/sensor/bathroom/atc_humidity
/node-name/sensor/bathroom/atc_battry-voltage
/node-name/sensor/bathroom/atc_battry-level
/node-name/sensor/bedroom/atc_temperature_
/node-name/sensor/bedroom/atc_humidity
/node-name/sensor/bedroom/atc_battry-voltage
/node-name/sensor/sbenoom/atc_battry-level

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

The mqtt label hierarchy is more clear in this way.

**Additional context**

Start by tracing how esp32_ble_tracker values are turned into MQTT topic labels. Compare the current flat examples with the requested sensor-name hierarchy, and define done as producing the requested nested topics without losing the existing sensor values or labels.

Written by the indexing model from the issue text.