More testing for CLI usage of Ed25519 and Ed448 keys

More testing for CLI usage of Ed25519 and Ed448 keys

Add testing for the `req` app and explicit conversion tests similar to
what is done for ECDSA keys.

The included test keys for Ed25519 are from the examples in RFC 8410
(Sec. 10)

The key for Ed448 is derived from the first of the test vectors in
RFC 8032 (Sec. 7.4) using OpenSSL to encode it into PEM format.

(cherry picked from commit 81722fdf2e01cfa71c46abbcc19e65aa003e083f)

This is originally a cherry-pick from
https://github.com/openssl/openssl/pull/10410, with trivial changes from
the original commit to account for the differences in 1.1.1.

Fixes #10687

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11939)