kvssink .so with static linkage to cproducer

[mateuszboryn]

Issue #, if available: #1178

Description of changes:
Adding cmake option BUILD_GSTREAMER_PLUGIN_STATIC to allow building GST plugin as .so with statically linked deps.

[disa6302]

I dont think it is still clear to me from the description why we need a new option. We could repurpose BUILD_STATIC for this. What is the issue when using BUILD_STATIC vs BUILD_GSTREAMER_PLUGIN_STATIC? Because I do not see any other changes other that this new flag.

[mateuszboryn]

With existing implementation, if BUILD_STATIC is ON, then libgstkvssink is always built as a static library (which makes little sense for gstreamer usage). This new option allows to set BUILD_STATIC to ON, while building libgstkvssink as a shared library.
It's not perfect, but at least allows building libgstkvssink.so that has libcproducer built-in.

[hassanctech]

The naming here is confusing and in the comments it says "If building GStreamer plugin, build it as a static library"

But what you're saying is you don't want to build it as a static library, rather you want to build as shared object and the shared object should in turn have it's producer c dependency statically linked in. We should be explicit about what it does. I am on board to have a build option that statically links in the producer library, but the plugin itself is still a shared object so we need the naming of such an option should be clear.