PEM_write_ed25519

[Anabelle VanDenburgh]

For no particular reason, i'm looking to convert my ed25519 private key 
to pem. A user on stackoverflow suggested running `ssh-keygen -p -f 
path/to/your/key -m pem` [^1], which errors, printing 
`do_convert_to_pem: unsupported key type ED25519`. Looking at the 
corresponding line in source, it seems that only RSA, DSA, & ECDSA keys 
can be converted to pem.[^2] With new keys defaulting to ED25519 as of 
[1.471][^3], are there plans to add support for exporting ed25519 to pem?

- Anabelle

[^1]: <https://stackoverflow.com/a/69207820/13224318>

[^2]: ssh-keygen.c, line 385: 
<https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-keygen.c?annotate=1.472&only_with_tag=MAIN#385>

[^3]: 
<https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-keygen.c?rev=1.471&content-type=text/x-cvsweb-markup>