mariadb/plugin/auth_ed25519

This plugin uses public domain ed25519 code
by Daniel J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, Bo-Yin Yang.

It is "ref10" implementation from the SUPERCOP:
https://bench.cr.yp.to/supercop.html

OpenSSH also uses ed25519 from SUPERCOP, but "ref" implementation.

There are four ed25519 implementations in SUPERCOP, ref10 is faster then ref,
and there are two that are even faster, written in amd64 assembler.
Benchmarks are here: https://bench.cr.yp.to/impl-sign/ed25519.html

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MariaDB changes:

API functions were simplified to better fit our use case:
* crypto_sign_open() does not return the verified message, only the
  result of the verification (passed/failed)
* no secret key is generated explicitly, user specified password is used
  as a source of randomness instead (SHA512("user password")).
* lengths are not returned, where they're known in advance
  (e.g. from crypto_sign()).
* crypto_sign() does not take the public key as an argument, but
  generates it on the fly (we used to generate public key before
  crypto_sign(), doing it internally avoids double work).

See the changes done in this commit.