Bluetooth: RFCOMM: Add RPN command sending support

Add support for Remote Port Negotiation (RPN) commands in the RFCOMM.
This allows applications to configure the serial port parameters
over RFCOMM connections, such as baud rate, data bits, stop bits, parity
and flow control.

Changes include:
- Add enumerations for RPN parameters (baud rate, data bits, stop bits,
  parity)
- Add a public API function bt_rfcomm_send_rpn_cmd() to send RPN
  commands
- Add shell command to test RPN with default parameters

This functionality is required by certain Bluetooth profiles that use
RFCOMM and need to configure serial port parameters.

Signed-off-by: Jiawei Yang <jiawei.yang_1@nxp.com>