provisioning_client: build libprov_mqtt_transport.so

[besquivel-pivotal]

Adding support in provisioning_client/CMakeLists.txt to build an so library for libprov_mqtt_transport.

Checklist

• [Yes] I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-c/blob/main/.github/CONTRIBUTING.md).
• [No] I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • [No] I edited the corresponding document in the devdoc folder and added or modified requirements.
• I submitted this PR against the correct branch:
  • [Yes] This pull-request is submitted against the main branch.
  • [Yes] I have merged the latest main branch prior to submission and re-merged as needed after I took any feedback.
  • [Yes] I have squashed my changes into one with a clear description of the change.

Reference/Link to the issue solved with this PR (if any)

This addresses #2435.
In addition to adding build support for libprov_mqtt_transport.so, this will allow easier updates in Intel's meta-iot-cloud yocto layer.

Description of the problem

The build as dynamic config switch has no effect on the mqtt_transport library. While the build outputs a static library, there is no dynamic library being built, even if the -Dbuild_as_dynamic:BOOL=ON switch is specified.

Description of the solution

To solve this issue, I just looked at how the other so libraries are built in this SDK and added the build support for that.

Testing

1. built the sdk using the build_as_dynamic switch and verified that both (.a and .so) libraries were produced.
   /usr/lib/libprov_mqtt_transport.so
   /usr/lib/libprov_mqtt_transport.so.1
   /usr/lib/libprov_mqtt_transport.so.1.11.0
   /usr/lib/libprov_mqtt_transport.a
2. Verified that the so versioning was present (see build artifact output above)
3. Linked and used the libprov_mqtt_transport.so with my application.
4. Verified that my application can connect with the IoT Hub using this .so.