Add pairing support to BTProxy

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

To be able to pair with BLE devices from the BTProxy, even if the pairing pin has to be entered via YAML.

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

The Bluetooth EQ3 Thermostats integration require active and paired connections and they stop working as soon as a BTProxy has better RSSI than the local adapter.

**Additional context**

This is the custom EQ3 Thermostat integration I am using: https://github.com/dbuezas/eq3btsmart
The integration in HA's core has been broken for a couple of months so I made this fork that brings back support, adds discovery and config-flows, all missing entities and services to change the internal heating schedules. I intend to publish the new python library and make a PR to the HA core once the incompatibility with BTProxy is solved.

I gave it a shot to implement pairing in esphome but did not succeed. I understand HA doesn't support it yet, but adding pairing pin via yaml would already be super useful in these cases :)