make BLEMidiServer.sendMessage() public

Please make the `BLEMidiServer.sendMessage()` method public, so we can send messages like this:

```c++
uint8_t midiMessage[] = {
    0xF0, // sysex
    0x00,
    0x21,
    0x3C,
    0x00,
    0x10,
    0x33,
    0x60,
    0x5A,
    0x00,
    0x7F,
    0xF7 // end of sysex
};
BLEMidiServer.sendMessage(midiMessage, sizeof(midiMessage));
```