Currently only covers what is needed to keep code compiling, plus
everything caps/structure/tags related.
This avoids unnecessary heap allocations for adding the NUL-terminator
of C strings, and especially makes caps/structure handling as efficient
as in C.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1190>
AudioCapsBuilder::new() will have the default values for
rate/channels/layout/format. Similarly, VideoCapsBuilder::new() will
have the default values for format/width/height/framerate.