design/68723-crypto-ssh-v2.md: new proposal

For golang/go#68723
Updates golang/go#65269

Change-Id: Ied7a96ab990abe257c4b8c295e292f92a745a4a7
GitHub-Last-Rev: 33f25dd
GitHub-Pull-Request: #51
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/602855
Commit-Queue: Nicola Murino <[email protected]>
Reviewed-by: Nicola Murino <[email protected]>
Auto-Submit: Nicola Murino <[email protected]>

Author: drakkan