IRK Sensor/Onboarding for Private BLE

**Describe the problem you have/What new integration you would like**
Let esphome provide the IRK (Identity Resolving Key) for devices to be used in the Home Assistant [Private BLE](https://www.home-assistant.io/integrations/private_ble_device) integration.

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

Private BLE needs the IRK of a device, which apparently can be determined by peering a (e.g, android) device using bluetooth. With private BLE, esphome often is already present as bluetooth proxy, so the "onboarding" of a new device might be useful here also.

espresense as alternative firmware already does this (https://www.home-assistant.io/integrations/private_ble_device/#espresense), but needing another esp just to read out the IRK, while the remaining parts of the presence tracking are done by Private BLE/esphome bluetooth proxy (instead of espresense) is not straightforward.

Read the linked Home Assistant Private BLE integration documentation and compare the ESPresense behavior referenced in the issue. Define how ESPHome Bluetooth proxy onboarding would obtain and expose a device's IRK, and consider the integration boundary before implementation; done means a new device can be onboarded without requiring a separate ESPresense device.

Written by the indexing model from the issue text.