[add] a method to extract certs and keys from a .p12 file

[pizzasaurusrex]

What would you like to be added

Add a step command to extract a crt and key from a .p12 file

Why this is needed

step certificate p12 can package a cert and keys to a .p12 file but there is not currently a way to extract the files once they are packaged.

[majd102-p]

I have submitted PR #1620 to address this. It implements the p12 extract subcommand as requested. Looking forward to getting this merged!