feat: add cli option to enable serial hardware flow control

[beniaminopozzan]

This PR adds the CLI option "-fc/--flow-control" to enable hardware flow control for serial and multiserial connections.

The default behaviour is to have the hardware flow control disabled which is consistent with the current implementation. If the options -fc or --flow-control are provided then hardware flow control is enabled.

Just like the baudrate setting, the flow control one is applied to all serial connections in multiserial mode.

The PR also fixes the get_help() formatting, adding a missing new line character.

Before the PR

# MicroXRCEAgent -h
Usage: 'MicroXRCEAgent <udp4|udp6|tcp4|tpc6|canfd|serial|multiserial|pseudoterminal> <<args>>'

Available arguments (per transport):
  * COMMON
    -h/--help.
    -m/--middleware <value> (ced, dds, rtps) [default: 'dds'].
    -r/--refs <value>.
    -v/--verbose <value> ( - ) [default: ''].
    -d/--discovery <value> [default: '7400'].
    -P/--p2p <value>.
  * IPvX (udp4, udp6, tcp4, tcp6)
    -p/--port <value>.
  * SERIAL (serial, multiserial, pseudoterminal)
    -b/--baudrate <value> [default: '115200'].
    -D/--dev <value>.  * CAN FD (canfd)
    -D/--dev <value>.

After the PR

# MicroXRCEAgent -h
Usage: 'MicroXRCEAgent <udp4|udp6|tcp4|tpc6|canfd|serial|multiserial|pseudoterminal> <<args>>'

Available arguments (per transport):
  * COMMON
    -h/--help.
    -m/--middleware <value> (ced, dds, rtps) [default: 'dds'].
    -r/--refs <value>.
    -v/--verbose <value> ( - ) [default: ''].
    -d/--discovery <value> [default: '7400'].
    -P/--p2p <value>.
  * IPvX (udp4, udp6, tcp4, tcp6)
    -p/--port <value>.
  * SERIAL (serial, multiserial, pseudoterminal)
    -b/--baudrate <value> [default: '115200'].
    -fc/--flow-control.
    -D/--dev <value>.
  * CAN FD (canfd)
    -D/--dev <value>.

You can now run: MicroXRCEAgent serial --dev /dev/ttyUSB0 -b 921600 and observe that no flow control has been enabled:

$ stty -F /dev/ttyUSB0 -a
speed 921600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 1;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt -echoctl -echoke -flusho -extproc

And you can run: MicroXRCEAgent serial --dev /dev/ttyUSB0 -b 921600 -fc and observe that flow control has been enabled:

$ stty -F /dev/ttyUSB0 -a
speed 921600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 1;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread clocal crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt -echoctl -echoke -flusho -extproc