Enables support for custom function codes. Modbus specification allows
vendor specific function codes in the range 65-72 & 100-110 [1] and this
feature allows users to implement custom logic for those codes.
Additionally, since the Zephyr Modbus stack doesn't implement all defined
Modbus fcs this feature allows users to add support for codes outside the
basic register reading / writing functionality offered by Zephyr.
Custom function codes can be added on a per-interface basis and the handler
structures are allocated by the caller.
[1]: https://modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf
Signed-off-by: Henrik Lindblom <henrik.lindblom@vaisala.com>
01757cfd32