feat: Add OdbBackend::open_writepack interface

Added `OdbBackend::open_writepack`, for opening streams that write
packfiles.

Added `OdbWritepack` trait for custom Writepack implementations.

Added an associated type `OdbBackend::Writepack: OdbWritepack` that
tells git2 what data to include for writepack implementations.

Implemented `OdbWritepack` for `Infallible` so that people can opt out
of implementing the Writepack trait.
The implementation panics when any method is called, though that would
be undefined behaviour since it would require `Infallible` to have been
instantiated.

A lot of types were duplicated in the process of this commit. I want
other people's thoughts on how to merge them before I make any changes
to their code, so I'll open a draft PR soon.

Author: tecc