spa: pod: Add wrapper for builder

Adds an (unsafe) wrapper around spa_pod_builder.

I designed this one to always use a Vec<u8> that the builder resizes as needed, I wasn't able to get it to be able to work with both an unresizable slice as well as a resizable Vec very well.

TODO:

1. Add missing methods All important methods have been added, a future MR might add the rest
2. Write documentation Safety sections have been written, more detailed docs could be done in the future.