MIDI over BLE support for ESP32, ArduinoBLE and Raspberry Pi Pico W

[tttapa]

This set of changes completely refactors the high-level BluetoothMIDI_Interace implementation to have it support many different backends using different BLE libraries. Previously, only the Bluedroid stack on ESP32 was supported.

The following backends are now supported:

• ESP32 Bluedroid (mostly untouched, simply updated the interface for callback functions to use the new BLEAPI)
• ESP32 NimBLE (new)
• ArduinoBLE (new, with some limitations)
• Pi Pico W BTstack (new)

To make use of MIDI over BLE, add #include <MIDI_Interfaces/BluetoothMIDI_Interface.hpp> to your sketch.
The BluetoothMIDI_Interface API remains mostly the same, except for the flush() function, which has been removed in favor of sendNow(), for consistency with the other MIDI interfaces. The original BluetoothMIDI_Interface tests have been ported, and new tests for some of the low-level BLE packet buffering functions have been added.

The documentation now includes a MIDI over BLE page with details about the different backends.
An example named BLEMIDI-Adapter.ino has been added.