`compile-examples.yml` does not cover Modbus MD02 demo

[aliphys]

According to the README.md for the opcua_server.ino sketch, additional flags need to be included in order for the Modbus MD02 example to compile
https://github.com/bcmi-labs/Arduino_open62541/blob/0783872bcb2739652ecafa70a07273271425e244/examples/opcua_server/README.md#L6

Current behavior only covers the compilation without the build flag. However, currently the build fails when the build flag is applied (see #12 ).

Two solutions:

• The compile-sketches.yml workflow is modified to compile the example twice, once with and another without the build flags.
• The opcua_server.ino sketch is split into two sketches