Getting public key from .PEM file for ostree sign verify

[MR ZenWiz <mrzenwiz gmail com>]

I have been digging around to find out how to do this, but it appears
all the methods I've seen so far produce text-converted keys.

How do I extract the binary public key from a .pem file so I can use
ostree sign to verify the signature in the commitmeta?

I'm not new to openssl in general, but this level of specific detail
is new to me.

Many thanks!

Mark