chore: add metadata.h

[mgerb]

I'm using this in one of my projects and I needed the metadata extensions. There are a few more headers in the extensions directory, but I didn't bother adding them because I don't need them at this time. May want to consider adding them when this hack is removed.

I added the following to the video example to generate the bindings. I then copied the metadata specific code into c.zig.

const c = @cImport({
    @cInclude("spa/utils/result.h");
    @cInclude("spa/param/video/format-utils.h");
    @cInclude("spa/param/tag-utils.h");
    @cInclude("spa/param/props.h");
    @cInclude("spa/param/latency-utils.h");
    @cInclude("spa/debug/format.h");
    @cInclude("spa/debug/pod.h");
    @cInclude("pipewire/pipewire.h");
    @cInclude("pipewire/extensions/metadata.h");
});

[MasonRemaley]

Whoops, sorry for letting this sit for so long! I apparently had notifications off on this repo by mistake. Thanks for the PR!