Support gstreamer

[jf2048]

Need to write an example using the macro with an gstreamer element, and possibly create another gst_element macro to make it easier.

[thiblahute]

I implemented a webrtcsrc element based on this crate, I think we have mostly everything we need tbh.

[thiblahute]

I also started adding macros to ease GstElement implementation here, at this point it implements element metadata and pad templates declaration in the macros like:

#[gobject::gst_element(
    class(final),
    long_name = "TheTestElement",
    classification = "Test/Filter",
    description = "Just a test",
    author = "Thibault Saunier <tsaunier@igalia.com>",
    pad_templates(
        src(presence="always"),
        // `__` is transformed to `_%` as "%" is not a valid character
        sink__u(direction="sink", presence="sometimes", caps="video/x-raw"),
    )
)]

My next step will be to improve the way we interact with Pads creation and integration into the Elements, maybe adding a macros like:

    #[gobject::gst_pad(sink__u)]
    impl SrcPad {
        fn chain() -> Result<gst::FlowSuccess, gst::FlowError> {
            ...
        }
    }

but I am still thinking.