Add design for velero backup performance improvements

[sseago]

Thank you for contributing to Velero!

Please add a summary of your change

This PR adds a design document to enhance single backup performance with multithreaded item backup. This design will also implement some of the prerequisites for future support of VolumeGroupSnapshots.

Does your change fix a particular issue?

Fixes #7474

Please indicate you've done the following:

• [x ] Accepted the DCO. Commits without the DCO will delay acceptance.
• [x ] Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• [ ] Updated the corresponding documentation in site/content/docs/main.