Rebase datalad-annex Git remote helper on git-annex

[mih]

As a result of https://distribits.live (2024), there is now a git-annex TODO to provide functionality analog to this feature, but generalized and more efficient: https://git-annex.branchable.com/todo/git-remote-annex/

Once available, the datalad-annex remote helper can be gutted and rebased on this functionality. We likely need to continue to provide the current implementation (for some time), or come up with a good-enough mapping.

Arguments that would make people "upgrade":

• git-bundle based implementation is more standard, and more efficient (incremental pushes, etc)
• current implementation can lead to data unavailability when a push is interrupted