Adds a dev_sign module to sign a root metadata with development keys.
No-Docs-Update-Reason: module in early development.
Change-Id: I3417ab711968d4f8ee1bcf9f164d9730d53af1b7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/62901
Reviewed-by: Joe Ethier <jethier@google.com>
Commit-Queue: Ali Zhang <alizhang@google.com>