modbus_rtu_lib: implement library APIs

Implement modbus-rtu library APIs which will be used by
phosphor-modbus-rtu service based on [1].

[1]: https://gerrit.openbmc.org/c/openbmc/docs/+/77318

Test

modbus_rtu_lib: implement library APIs

Implement modbus-rtu library APIs which will be used by
phosphor-modbus-rtu service based on [1].

[1]: https://gerrit.openbmc.org/c/openbmc/docs/+/77318

Tested:
Added a Mock Modbus RTU server using socat which intercepts and replies
to modbus messages for testing.
```
> meson test -C builddir
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/2 test_modbus_commands OK 0.01s
2/2 test_modbus OK 6.02s

Ok: 2
Fail: 0
```

Change-Id: I66cdc8fd930dd6f7ad6888116d1419ad8f8b8ed8
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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