WOLFMQTT_EXAMPLE_CERT allowing static or extern cert assignment

[gojimmypi]

Related to #352, this PR introduces the WOLFMQTT_EXAMPLE_CERT to allow the CMakeLists.txt in an Espressif Managed Component project to build the AWS example using the certs defined in examples/aws/awsiot.c and used in the examples/mqttexample.c source.

GitHub Workflow Update Included

edit: Thank you @bandi13 for the .github/workflows/ubuntu-check.yml update included here that helped identify the occasional workflow network failure, as well as the workflow updates in #355.

Background

Without this change, included files outside of the project directory like this:

idf_component_register(SRCS
                       	    main.c
                       	    wifi_connect.c
                       	    time_helper.c
                       	    ${WOLFMQTT_ROOT}/examples/aws/awsiot.c
                       	    ${WOLFMQTT_ROOT}/examples/mqttexample.c
                       	    ${WOLFMQTT_ROOT}/examples/mqttnet.c
                       	    ${WOLFMQTT_ROOT}/examples/mqttport.c
                        INCLUDE_DIRS
                            "."
                            "./include"
                        )

... fail to properly build:

C:/workspace/wolfMQTT-gojimmypi/examples/mqttexample.c:662:30: error: 'device_cert' undeclared (first use in this function)
  662 |                 (const byte*)device_cert, (long)sizeof(device_cert),

The PR is an alternative to my current method of copying those files to the local project directory. Although the copy works, it is one more point of maintenance when the example certs expire.

Usage

Define WOLFMQTT_EXTERN_CERT either in user_settings.h, or in the case of Managed Component examples, this cmake line is added to the main project CMakeLists.txt:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DWOLFMQTT_EXTERN_CERT")

See also the published wolfSSL Managed Component.

[gojimmypi]

retest this please

I checked this PR on Ubuntu,: installing wolfSSL and successfully doing a make for all the MQTT examples. I'm not sure why the tests failed with Error: The operation was canceled.

[embhorn]

Great!

[embhorn]

Please fix the conflicts after I merged #355

[gojimmypi]

Jenkins retest this please.