liburl: minor api changes

See commits for info, a number of these are 'breaking', although liburl is marked experimental so I'm not sure that matters so much.

First two commits will be impacted if #15138 is adopted, but it's a simple rename.
We've starting to see more pickup of `BytesContainer` in #15280 for example, so I think my personal preference may be overruled by ergonomics (this is fine as is)