When implementing NDK media support, it would be useful to also have JNI
implementation in the same binary as NDK media compatibility is lower.
As such, implement a rudimentary vtable system for gstamc-codec and
gstamc-format, and allow choosing the implementation at static_init()
time.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4115>