Add tx callback to cdc device

Useful for continuous transmission of data, which is difficult currently
because there is no notification of tx completion.

src/class/cdc/cdc_device.h

@@ -128,6 +128,9 @@ TU_ATTR_WEAK void tud_cdc_rx_cb(uint8_t itf);
 // Invoked when received `wanted_char`
 TU_ATTR_WEAK void tud_cdc_rx_wanted_cb(uint8_t itf, char wanted_char);
 
+// Invoked when space becomes available in TX buffer
+TU_ATTR_WEAK void tud_cdc_tx_cb(uint8_t itf);
+
 // Invoked when line state DTR & RTS are changed via SET_CONTROL_LINE_STATE
 TU_ATTR_WEAK void tud_cdc_line_state_cb(uint8_t itf, bool dtr, bool rts);