Add EVP demo for X25519 key exchange

This offers both a known answer test with fixed keys and also
demonstrates a more realistic usage with random keys.

Fixes #14118.

R

Add EVP demo for X25519 key exchange

This offers both a known answer test with fixed keys and also
demonstrates a more realistic usage with random keys.

Fixes #14118.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17799)

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