Metadata for pdfs - cairo_pdf_surface_set_metadata()

[johnomotani]

Would it be possible to add a wrapper for cairo_pdf_surface_set_metadata() (https://www.cairographics.org/manual/cairo-PDF-Surfaces.html#cairo-pdf-surface-set-metadata)?

I think this would be useful to enable MakieOrg/Makie.jl#3167.

I would be willing to have a go at this, but have no experience using ccalls or developing Cairo.jl - would it be quicker for someone who knows what they're doing to just add this than it would be to review a PR?