Add a message to query the transport buffer size

[soundanalogous]

It would be helpful to have a way to query the transport buffer size. This is typically 64 bytes for Serial, but can vary based on board architecture. The buffer size for other transports such as BLE and TCP may vary as well. The size is typically exposed as a constant in firmware so it should be possible to provide a Firmata message to query the size. This will be helpful for client libraries in determining where to break up large data buffers prior to transmission.