1313: (embassy-stm32): rework bufferedUart to get rid of PeripheralMutex r=Dirbaio a=MathiasKoch
New implementation is very similar to the implementation of embassy-nrf & embassy-rp.
Also adds embedded-hal traits to bufferedUart.
**NB**: Still needs testing on actual hardware
Co-authored-by: Mathias <mk@blackbird.online>