Adds support for importing ECDSA (NIST-P256) public keys from standard
pem files as KeyMapping objects.
Also refactors key generation to persist keys in standard pem formats
instead of customized JSON format. The pem formats are much more
recognizable in general and also removes the need for the
securesystemslib module.
No-Docs-Update-Reason: module in early development
Change-Id: Ie06808728137e70e35afdceb5254cf6abf20f1ea
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/62441
Commit-Queue: Ali Zhang <alizhang@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
