rtu: getConfig shall return unique_ptr

Update the getConfig in PortFactory and related classes to return a
unique_ptr instead of a variable on stack to preserve polymorphic
behavior of the return po

rtu: getConfig shall return unique_ptr

Update the getConfig in PortFactory and related classes to return a
unique_ptr instead of a variable on stack to preserve polymorphic
behavior of the return port config. Without this change the values from
USBPort are dropped when casting from USBPort to PortFactoryConfig.

Change-Id: Ia250771a7fd7aedb133598446aab1e120aaf36b5
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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rtu: add serial port interface

Add the interface classes for serial port with Port Factory classes to
make the code extensible for future in case a new hardware port type is
introduced. This also ma

rtu: add serial port interface

Add the interface classes for serial port with Port Factory classes to
make the code extensible for future in case a new hardware port type is
introduced. This also makes the unit testing easy by creating a Mock
Port using socat.

Tested:
```
meson test -C builddir test_port
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/1 test_port OK 5.02s

Ok: 1
Fail: 0
```

Change-Id: Ic6bd982abf1ae993f76c39e3503d3a0402a692fe
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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