Fix the issues #10501 and #9733
If having gearbox, we need:
* add gbsyncd as a peer since swss also has dependency on gbsyncd
* add service gbsyncd to FEATURE table if it is missing
This change is to add a gbsyncd container to accommodate the syncd process and the SAI libraries for the Credo gearbox chips.
How I did it
This container works similar to the existing Broadcom syncd container. Its main difference is that the SAI-related dynamic libraries are replaced by the ones for Credo gearbox chips, and the container only reacts to SAI events for the gearbox chips. The SAI libraries will be provided by the package libsai-credo_1.0_amd64.deb.
For the image build, the added container will be built and included in the Broadcom platform image, after $(LIBSAI_CREDO)_URL = is replaced to the correct value. For now, as $(LIBSAI_CREDO)_URL is empty, the container build is skipped in the image build.
After the container is included in the image, in the runtime, the container will begin with checking the existence of /usr/share/sonic/hwsku/gearbox_config.json; if that file is not provided, the container will exit by itself. Therefore, for platforms unrelated to the Credo chips, as long as they are not providing the file, they will not be affected by this change.
