repo: add some newer RSA/ECC signing keys

We've been using a DSA/1024 key to sign our tags.  Time to update to
something a bit newer.  We'll include RSA & ECC keys, but only use
RSA keys initially for backwards compatibility and see how it goes
with our user base.

Change-Id: I683c97b6fbd860f220ed4ddc7b21f07db279a916
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255112
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>