SITL: add UART tx/rx tracking and @SYS/uarts.txt

[IamPete1]

Extract from #26532. This adds a byte tracking helper StatsTracker which allows the UART driver to keep a cumulative count rather than zeroing. ChibiOS is moved to use that and SITL gets by tracking and fills in @SYS/uarts.txt.

SITL:

UARTV1
SERIAL0 TX=   60956 RX=     354 TXBD= 36742 RXBD=   213 connected     (tcp:0:wait)
SERIAL1 TX=       0 RX=       0 TXBD=     0 RXBD=     0 not connected (tcp:2)
SERIAL2 TX=   19259 RX=     812 TXBD= 11608 RXBD=   489 connected     (tcp:3)
SERIAL3 TX=    1089 RX=   35358 TXBD=   656 RXBD= 21312 connected     (GPS1)
SERIAL4 TX=       0 RX=       0 TXBD=     0 RXBD=     0 connected     (GPS2)
SERIAL5 TX=       0 RX=       0 TXBD=     0 RXBD=     0 not connected (tcp:5)
SERIAL6 TX=       0 RX=       0 TXBD=     0 RXBD=     0 not connected (tcp:6)
SERIAL7 TX=       0 RX=       0 TXBD=     0 RXBD=     0 not connected (tcp:7)
SERIAL8 TX=       0 RX=       0 TXBD=     0 RXBD=     0 not connected (tcp:8)

ChibiOS (CubeOrange):

UARTV1
SERIAL0 OTG1  TX =    7809 RX =     620 TXBD=  8632 RXBD=   685
SERIAL1 UART2 TX =     214 RX*=       0 TXBD=   236 RXBD=     0
SERIAL2 UART3 TX =     214 RX*=       0 TXBD=   236 RXBD=     0
SERIAL3 UART4 TX =    1296 RX =       0 TXBD=  1432 RXBD=     0
SERIAL4 UART8 TX =       0 RX*=       0 TXBD=     0 RXBD=     0
SERIAL5 UART7 TX =     611 RX*=     234 TXBD=   675 RXBD=   258
SERIAL6 OTG2  TX =       0 RX =       0 TXBD=     0 RXBD=     0
IOMCU   UART6 TX*=   72638 RX*=   42920 TXBD= 80298 RXBD= 47446

The last commit updates the SITL bw_in_bytes_per_second function to report the true baudrate if baud rate limiting is enabled.

[MattKear]

Idea was raised (not for this PR) that it would be good to add logging for these roughly every 10s in flight.

[tridge]

@IamPete1 great work, thanks!