modbus_rtu_lib: define read holding register

Add the modbus rtu library commands with initial support for read
holding register. Define a base Message classes which all subsequent and
specific reque

modbus_rtu_lib: define read holding register

Add the modbus rtu library commands with initial support for read
holding register. Define a base Message classes which all subsequent and
specific request & response messages can inherit from. Also add the
relevant unit testing for the added command set.

Tested:
```
> meson test -C builddir
ninja: Entering directory `/host/repos/Modbus/phosphor-modbus/builddir'
ninja: no work to do.
1/1 test_modbus_commands OK 0.01s

Ok: 1
Fail: 0

```

Change-Id: I331b0dee66a0829e9352ae0eac8ac82a9150904c
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>

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