Investigate refactoring how we return AVIOFileLikeContext

[scotts]

PR #889 refactored how we create decoders that take a file-like object. See the PR for full details. The state the code was left in, though, still has a not-great quirk that we're doing this:

1. C++ creates object in pybind_ops.cpp, does a reinterpret_cast<int64_t> and returns that value to Python layer.
2. Python layer accepts the int and forwards that int decoder creation in custom_ops.cpp.
3. C++ in custom_ops.cpp does a reinterpet_cast<AVIOFileLikeContext*> on the int.

That's not great. The purpose of this issue is to investigate if defining an actual pybind object that wraps AVIOFileLikeContext is better.